Lagar received his early training in the workshop of his father, a cabinetmaker who specialized in religious commissions. In 1907 he moved to Madrid to study with the sculptor Miguel Blay, and from 1910 he continued his studies at the Escuela de la Llotja in Barcelona. The following year he settled in Paris thanks to a scholarship, and initially concentrated on sculpture. He came into contact with Modigliani and the Cubist painters, including Metzinger, and also met his future wife, the French sculptor Hortense Begué.
He held his first solo show at the Ashnur Gallery. Between 1914 and 1916 he moved several times: first living in a town in the French Pyrenees, then in Girona, and finally in Blanes. The Dalmau Galleries held his first solo exhibition in Barcelona in 1915, also featuring Hortense Begué’s work, and he was praised by Eugeni D’Ors in his Spanish review. During this decade he took part in exhibitions both in the Catalan capital, along with his wife, at the Galeries Laietanes in 1916 and 1918, and in other cities such as Girona, Madrid, Bilbao, and Orense. He became friends with Junoy and Barradas. To refer to his work from this period, in which the influence of Cubism and Futurism was evident, the artist himself coined the term ‘Planism’ which, although it only had a vague theoretical definition given by Lagar, found some popularity among the critics of that time.
In 1915, the magazine Cultura published his article entitled The Rebirth of Art after Cubism. His graphic contributions appeared in Un enemic del Poble, Revista Nova and Troços. Back in Paris, he showed his work assiduously; among others, he exhibited at the Berthe Weil Gallery in 1919 and 1922, and the Percier Gallery in 1924. The Druet Gallery also supported his exhibition in 1935 with a catalog written by D’Ors and translated by Cendrars.
The circus motifs, a common feature of his work from the 1920s, are treated in a way that reveals the influence of Picasso. This theme was joined by the landscapes of Normandy and evocations of Spanish life and art, and by the following decade, the world of bullfighting. In 1936, he participated in the Society of Iberian Artists exhibition held in Paris. During the Spanish Civil War, he supported the Republican side. At the end of the Second World War, he resumed his exhibitions in Paris and other French cities. When his wife died in 1956, the artist abandoned painting and spent a long period of time in seclusion in the Sainte-Anne hospital before moving to Seville in 1964, where he died two years later.