The son of a miller, Munnings was born in Mendham on the Norfolk-Suffolk border. He was educated at Redenhall Grammar School and Framlingham College. At age 14, he was apprenticed for six years to the firm of Page Brothers, lithographers, in Norwich where he was noticed by John Shaw Tomkins, a director of Caley's Chocolates, who became his earliest patron. In 1902 he went to Paris to study at Julian's Academy for a few weeks, a practice he repeated several times. He also attended Frank Calderon's School of Animal Painting in Finchingfield, Essex. A passion for horses and horse painting led him to acquire the first of a succession of horses, both to ride and paint. He also visited race meetings and gypsy horse fairs.

 In 1910 Munnings moved to Cornwall, which he had already briefly visited, arriving in Newlyn with a group of girl models and soon becoming the life and soul of the artist community in Lamorna. A converted mill served as both studio and stabling for his horses. He became infatuated with a young art student, Florence CARTER-WOOD, who modeled on horseback for him; they were married in 1912, but this disastrous marriage ended with her suicide in 1914.

Records do not show that he ever exhibited in galleries in Cornwall, and since his death his work has been included in only one Cornish exhibition, as promoted by the Royal West of England Academy in Bristol (1992) Artists from Cornwall. However, since there is no list of exhibits in the accompanying exhibition catalogue, it is not known what work(s) of his were chosen. 

A five day stay at Mrs. Griggs' at Zennor in 1913 resulted in many fine paintings of his horses and groom, Ned Osborne. In 1917 he joined the army and a year later was sent to France as a war artist for the Canadian Cavalry, resulting in more paintings and memorials. Never afraid to show his dislike for modern art, at the opening of the Stanhope FORBES Memorial Exhibition in Newlyn (1949), he urged those attending 'to march' on the Tate as Forbes' The Health of the Bride was languishing in the basement while "absurd paintings by Matisse were hung preciously on the walls with half a mile between each one".

In 1919 Munnings bought Castle House in Dedham, Essex, and the following year Munnings married his second wife, Violet, also a horsewoman. He died in Dedham in 1959, aged 80.

Munnings became president of the Royal Academy in 1944. An outspoken critic of modernism, he gave a speech in 1949 at a Royal Academy banquet condemning modern art as 'damned nonsense'. This damaged his reputation considerably, but his landscapes continue to be highly regarded.