The pursuit of Lee began, but he was too swift for Meade, and escaped across the Potomac. After various movements on both sides of the Potomac, which was crossed July 17th, the 124th bivouacked at Manassas Gap July 22nd, in sight of the place of its first skirmish. Then the regiment had 700 rank and file, a full field staff and twenty-five line officers. Now, although about thirty convalescents had returned, it had less than 200 men in all. The corps, commanded by Major General French had been strengthened since Gettysburg by about 8,000 new troops. General Ward still commanded the division.
There were some undecisive {sic} movements and skirmishing in which the 124th participated. Lieutenant Colonel Cummins, having recovered, returned and took command of the regiment July 28th. It was soon established in camp near Sulphur Springs and remained there six weeks. September 10th, in consequence of the backward movement of Lee's army, General Meade pushed a heavy cavalry force across the Rapidan, and soon the entire army was transferred to the region between the Rapidan and Rappahannock. The 124th broke camp September 15th, and next encamped near Culpepper. October 2nd Lieutenant Colonel Cummins was given a leave of absence for five days, and left Captain Weygant in command. There were some uncertain movements, one of which followed a retrograde movement of Lee, October 19th, but he was found to be out of reach and General Meade gave up the pursuit until twenty miles of railroad could be re-built. Next came the fight at Kelly's Ford of November 7th, in which the 124th supported the 10th Massachusetts Battery, and the Confederates were defeated. Ward's division took possession of the ford and captured over 500 prisoners. The Union loss was only about fifty. Sedgwick's right wing attacked works on the bank of the Rappahannock, carried them by a brilliant coup de main, and captured 1,500 prisoners, his loss being about 300. In the march next day toward Beverly Station, with Ward's division in front, the 124th was deployed as flankers and was under fire for some time. There was no more fighting until November 27th, when the fight at Locust Grove took place, followed by that at Mine Run. The 124th took part in both, and lost one killed, eight wounded and three captured. December 1st Meade ordered a retrograde movement, and the regiment settled down to a long stay at Brandy Station and Culpepper. March 17 General Birney sent an order to Colonel Cummins stating that to equalize the brigades the 124th would be transferred to the Third Brigade. This was so displeasing that fourteen officers of the regiment petitioned General Birney to allow them to remain under Brigadier General Ward. Major General Birney replied that "the request of the officers of this gallant regiment would be fully considered," and he would try to grant it. This was done, and the 124th remained in Ward's Brigade. Meanwhile it had received considerable additions in volunteers from Orange County, although not half as many as had been mustered out and transferred to the veteran reserve corps in consequence of physical disability.
Early in March U. S. Grant had been made Lieutenant General and invested by the President with the chief command of all the armies of the United States. In the latter part of March and the month of April he caused a radical reorganization of the Army of the Potomac, and the "Old Third" became a part of the Second Corps.
An advance was ordered May 3d, and that night Birney's division struck tents and began its march. Two days afterward the memorable Battle of the Wilderness was begun. Ward's command in this battle consisted of eight infantry regiments, including the 124th and the Second U. S. Sharpshooters, constituting one of the largest brigades in the army. The leading officers of the 124th were now Colonel Cummins, Lieutenant-Colonel Weygant and Acting Major H. S. Murray. The brigade and regiment did their allotted share in the two-days' battle, but the regiment was less exposed than at Chancellorsville and Gettysburg, and came out with a loss of three killed and fifty-seven wounded, Colonel Cummins being among the wounded.
The battle of Spottsylvania Court House came a week later. On the 9th the 124th N. Y. and 20th Ind. Regiments were thrown out as skirmishers for General Birney's division as it marched forward. Near Po River a Confederate battery opened on them, but fired too high, and soon had to flee. In the battle the next day for a while the 124th supported a battery on the extreme right of Hancock's line, and then marched with the division towards the left, where it was soon engaged, and assisted in an unsuccessful charge on the rebel line. It was in General Hancock's famous charge of May 12th. Birney's division was in four lines, with Ward's Brigade in front, the 124th composing the right center of the first line, Lieutenant Colonel Weygant in command. When the clash came there were hand-to-hand encounters so fierce and desperate as to defy description. There was an "unparalleled struggle of eighteen hours' duration." Hancock's men captured the enemy's works, and he finally abandoned his efforts to retake them. The 124th had been so actively engaged or under fire so much that the men writing home as late as May 18th spoke of the battle as having been raging since May 4th. Its losses were fifteen killed, fifty-two wounded and two captured. Colonel Weygant was one of the wounded. After the battle the regiment was so small that it was found necessary to consolidate it into five and then into three companies, and the 124th and 86th acted together, first under Lieutenant Colonel Lansing and then Major Stafford, both of the 86th. This union lasted until they settled down in front of Petersburg.
On the evening of May 20th the movement of the army from Spottsylvania Court House began in earnest. The 124th was in various movements, and on June 1st seven of its men on picket duty were captured. In the bloody battle of Cold Harbor, June 3d, Birney's command was in reserve, and the men of the 124th were lookers-on.
Offensive operations were suspended for a time, and General Grant decided to transfer his army to the James River. On the evening of June 12th, after a march of fifty-five miles, Hancock's corps reached Wilcox Landing and was transferred to waiting steamboats. On June 15th the advance of the rest of the army was resumed, Birney's command leading the column down the Prince George Court House road towards Petersburg. The brigade encamped in a grove of pines on City Point. There were only about 100 men left in the 124th. Between May 18th and June 22nd it lost three killed, twenty-four wounded and eight captured. Every third day from the 4th to the 24th of July the remnant was on the picket line. There were no engagements, but some skirmishing.
General Birney had been assigned to the command of the Tenth Corps, and the veterans of the "Old Third" were consolidated into one division under General Mott, and General De Trobriand became commander of the brigade, which included the 124th, now commanded by Lieutenant Colonel Weygant. When offensive operations were resumed on the last day of September, the regiment could muster sixteen officers and 230 fighting men, and two months later had been reinforced so that it numbered nineteen officers and 362 enlisted men, a few of whom were raw recruits. It had some active and hazardous work during the fall, before the army settled down in winter quarters. October 26 Hancock's command moved to the Weldon road, and the next day De Trobriand's brigade marched with Mott's division to Hatcher's Run, where General Eagan was having a skirmish, and preparing to force the passage of the stream. This was done, and presently De Trobriand's brigade was ordered to the front to relieve the brigade on the left of Eagan's line. In the afternoon the enemy advanced in force, and there was some furious fighting, in which the 124th brought to a halt a flanking force of dismounted cavalry with a battery of rifled guns and sent them into the edge of a neighboring wood. The battle continued, and a bullet struck Colonel Weygant in the side, so that he had to leave the field. Finally the Confederates fled. This was called the Battle of Boydton road. Three of the 124th were killed, fourteen wounded and six captured including two of the wounded. The regiment also took part in the raid of a week, beginning November 6th, to destroy the Weldon railroad, when its loss was only one killed and one wounded.
During the winter the Union lines tightened around Richmond, but apart from preparations for the spring campaign not much was done on either side. In February however, De Trobriand's brigade was in a short fight at Hatcher's Run, across which the Union line was permanently extended.