Agate, the eldest child of Charles James Agate (1832–1909), a wholesale linen draper, and Eulalie Julia née Young, was born in Pendleton, near Manchester, England. His father had a keen interest in, and connections to, music and the theatre. Gustave Garcia, nephew to the prima donna Maria Malibran, was Charles's lifelong friend since they were apprenticed together in the cotton warehouse. Agate's mother was educated in Paris and Heidelberg, and was an accomplished pianist. Through Agate's family connections to the active German artistic community in Manchester, he had much exposure to performance in his youth. In October 1912, Sarah Bernhardt visited the Agate home, an indication of the family's position in the local arts scene. Agate's only sister, May, later studied acting under Bernhardt in Paris.
After education at Giggleswick School and Manchester Grammar School, where he was academically outstanding, he did not go to a university, but went into his father's business, where he worked for seventeen years. In his spare time he was a regular theatregoer, and admired and longed to emulate the critical writing of George Bernard Shaw in The Saturday Review. In 1906 he wrote a letter about drama to a local Manchester paper. The editor printed Agate's contribution and invited him to write a weekly theatre column. After a year Agate joined the Manchester Guardian's team of critics under the guidance of C. E. Montague. Even as a junior critic Agate did not hesitate to give bad notices to the leading figures of the English stage when he thought it justified. Within months of taking up his post, he wrote of Herbert Beerbohm Tree's performance as Richard II, "It was extraordinarily uninteresting, and it is amazing how badly a tragic part can fit an actor so fine as, in other directions, Mr. Tree undoubtedly is." Later, Agate was bested by Lilian Braithwaite, who responded to his assertion that she was "the second most beautiful woman in London" by replying, "I shall long cherish that, coming from our second-best theatre critic."
In his early twenties, Agate wrote a play, The After Years, which his biographer, Ivor Brown describes as "less than successfully realized". Another biographer, James Harding, said of Agate's subsequent attempts at fiction (a second play and three novels) that they are "of small import".
Agate volunteered in May 1915 at the age of thirty-seven for the Army Service Corps, and was posted to France. He had an arrangement to supply a series of open letters about his wartime experiences to Allan Monkhouse at The Manchester Guardian. These were published in his first book, L. of C. (Lines of Communication), of which a reviewer wrote, "Captain James E. Agate ranks as one of the first hundred thousand soldiers who have written a book about the war, but… one is sure there will be no other book like this one.… It is our old friend 'J. E. A.' at his irritating best in khaki." Agate's fluency in French and his knowledge of horses landed him a job as a hay procurer (described in the first volume of his Ego) in which he was outstandingly successful. His system of accounting for hay purchases in a foreign land in wartime was eventually recognised by the War Office and made into an official handbook. Captain Agate's name was engraved on the Chapel-en-le-Frith War Memorial in Derbyshire. After L of C, Agate published a book of essays on the theatre, Buzz, Buzz! (1918). In the same year, while still serving in France, Agate married Sidonie Joséphine Edmée Mourret-Castillon, daughter of a rich landowner. The marriage was short-lived, and after it broke up amicably, Agate's relationships were exclusively homosexual.
On returning to civilian life, Agate pursued his career as a theatre critic. In 1919 he published a second book of essays, Alarums and Excursions. In 1921 he secured the post with The Saturday Review once held by Shaw (and then by Max Beerbohm), and in 1923 he moved to The Sunday Times, where he remained theatre critic for the rest of his life. From 1925 to 1932 he combined his newspaper work with the post of drama critic for the British Broadcasting Corporation. His former paper, The Manchester Guardian, later wrote of him, "That Agate was the first dramatic critic of his time may well be doubted by adherents of Ivor Brown or Desmond MacCarthy, but beyond dispute he was the first theatrical critic. He was native to the theatre, he understood acting, he had in his blood both the French … and English stages."
In addition to his work as a theatre critic, Agate was film critic to The Tatler and literary critic to The Daily Express and also had leisure interests that occupied much time and money. He was a cricket and boxing enthusiast, the owner of Hackney show horses, and an avid golfer. All these are reflected in his diaries, published between 1935 and his death in a series of volumes entitled Ego, Ego 2, Ego 3, etc. (When he published Ego 8, his friend and sometime secretary Leo Pavia enquired, "Will the Ninth be choral?") The historian Jacques Barzun, a fan of Agate and editor of a reissue of the last two volumes of Agate's Ego series, highlighted Agate in 2001, which rekindled the interest of a new generation:
"When in 1932 he [Agate] decided to start a diary, he resolved to depict his life entire, which meant giving a place not solely to his daily thoughts and occupations but also to his talk and correspondence with others, including his brothers and sister, no less singular than himself. The resulting narrative, with fragments of hilarious mock-fiction, ranks with Pepys's diary for vividness of characterization and fullness of historical detail".
Alistair Cooke was another admirer of Agate, and devoted one of his "Letters from America" to the "Supreme Diarist."
Agate had a series of secretaries, of whom Alan Dent, known as Jock, who served for 14 years, was later the most prominent. Dent arrived on Agate's doorstep in September 1926: "He announced that his name was Alan Dent, that he resided at some absurd place near Ayr, that he had received university education, hated medicine and refused to be a doctor, that he admired my work, intended to be my secretary willy-nilly, and had walked from Scotland for that purpose. I looked at his boots and knew the last statement to be merely ad captandum and with intent to mollify." (From Ego , Page 91.)
Agate's style in the diary entries that constitute the nine volumes of "Ego" is discursive. Anecdotes of the day's news, excerpts from his voluminous correspondence with readers of his reviews and books, frank and often amusing ruminations on his health (he was a hypochondriac and obsessive-compulsive) and poor financial state abound. Many of his diary entries mention his friends Herbert Van Thal, George Lyttelton, Dent and Pavia, and Edward Agate, his much-loved brother. He had recurring themes around Malibran, Sarah Bernhardt, Réjane, Rachel, the Dreyfus Affair, Shakespeare, and Dickens. His style is "vigorous and outspoken, and always entertaining, in spite of his refusal to admit greatness in any actor later than Irving." He has been compared to critics of an earlier generation, Clement Scott and A B Walkley: "He admired the power Scott had enjoyed on the Daily Telegraph during the last third of the nineteenth century, and enjoyed Walkley's elitism and francophilia on The Times during the late Victorian and Edwardian periods. Agate sought to position himself in that tradition, and his criticism consequently is verbose and self-indulgent but hugely entertaining and revealing."
Agate made a short-lived and unsuccessful adaptation of a German play, I Accuse! from the original of Dr. Hans Rehfisch and Wilhelm Herzog; it opened and closed in London in October 1937. The Times reviewer commented, "Mr. Agate is suspected of having been too faithful to a too earnest German original". His theatrical notices were published in a series of collections including Buzz, Buzz!, Playgoing, First Nights, More First Nights, etc., and are valuable for their history of London theatre between the world wars. His anthology The English Dramatic Critics, 1660–1932 is important. He wrote a biography of the French actress Rachel, a heroine. Arnold Bennett called it an "excited and exciting biography" and "beyond question the best life in English" of the subject.
During World War II, Agate's health declined, and he began to suffer from heart trouble. He died suddenly at his home in Holborn, London, at the age of 69, shortly after completing his ninth Ego volume of diaries.