But in August of 1841 Leech began the great work of his life—a work, indeed, which he never quitted but with life—namely, his connection with Punch. The first number of Punch was issued on the 17th July, 1841, and Leech's first contribution to it appeared on the 7th August, in the fourth number. For about twenty years, it may be said, he was its leading spirit, and, by his contributions to its pages, got in all about £40,000. Political caricatures he produced by the score, and held up to ridicule many of the absurd customs of the pretensious and exclusive sections of Society. Like Thackeray and Dickens, Leech detested snobbery in all walks of life, and depicted it unsparingly in a way that it never had been dealt with before. Week after week there flowed from his pencil an endless stream of scenes of high life and low life, of indoor life and street life, now of England, and then of foreign lands, and of all times, seasons, and occasions, as also numerous scenes of deer-stalking and fishing, and of horses and hounds, in all cases depicting whatever he undertook with extraordinary accuracy combined with infinite humour. Also, when social or national wrong called for grave censure, Leech knew how to administer it, not only without giving unnecessary offence, but in the way best calculated to bring about reform and redress. In all circumstances he was essentially a humorist, and he found his most genial vocation in depicting life and character in the social circles he frequented. As a keen observer of the everyday life around him, he delighted to depict the corporation magnate, the artist, the medical student, the spendthrift, the policeman, the cab driver, the coster, the carman, and hundreds of other such phases of everyday life and character, seeing humour and drollery where others failed to observe anything but the commonest aspects of everyday monotony. Of course it should not be forgotten that, if Leech did great things for Punch, his connection with that journal gave him great opportunities, and brought him into the very forefront of British artists. He was considered the most successful humorist of the day, and his pencil was in constant request. In the course of years he became the illustrator of about eighty volumes. When it is realised that the sketches in Punch and the illustrations in these eighty volumes combined amount to some thousands in number, the mind is much impressed with the great amount of industry and application that Leech displayed throughout life. Even a tour to the Highlands, or to Ireland, or an outing to any portion of the country, was at all times turned to practical account for work later on.
This incessant brain-work produced an extreme nervous sensitiveness. In this state he was much affected by noise and was literally driven from his house in Brunswick Square by street music. He removed to Kensington, where he hoped to obtain a release from this annoyance by adopting a device of double windows. But he had no peace. He often introduced in the pages of Punch the barrel-organ nuisance. The public, however, at that time had no idea what these sketches from real life cost the artist. In 1864, Leech was ordered to take a holiday on the Continent. Upon his return to his London home in the autumn of the same year, although better in health, he was still strangely susceptible to noise. He spoke with more than his usual earnestness about the sufferings which the street organs gave him, and about the smallness of the sympathy which he received from people who had no weakness in the same direction. This extraordinary sensitiveness to noise was only a secondary phase or symptom of the real ailment. The real malady from which he suffered was breast-pang, or spasms of the heart, a form of angina pectoris. Although it was necessary to warn Leech against all excitement, riding, quick walking, or overwork, it was not supposed that he was in immediate danger, and, if he could only find rest and quiet, great hopes were entertained of his recovery. However, the sad end came when quite unexpected. In the morning of the 29th of October, 1864, he spoke hopefully of the future to his wife. In a few hours afterwards he whispered into the same living ear—"I am going," and fell into his father's arms in a faint. Three hours afterwards he expired. The news of his death went over the country with a dismal shock; for in what house was John Leech not an inmate in one form or another?
Leech was tall, with an elegant figure, over six feet in height, graceful and gentlemanly in manner, with a fine head and a handsome face. In action he was nimble, vigorous, and yet gentle, capable of the heartiest mirth, and yet generally quiet. He was singularly modest, both as a man and an artist. The perpetual going to nature kept him humble as well as made him rich. His consideration, too, for others was apparent at all times, and the gentleness of his nature was remarkable. When it is considered that all these beautiful traits of character were accompanied by such extraordinary talent and wisdom, one is profoundly impressed with the greatness of the man. No wonder so many mourned when such a great, gentle, and graceful spirit passed away. It was a national loss, and as such was realised throughout the homes of the United Kingdom.