Born into an artistic family, Weir was a founding member of the Ten American Painters and became one of the most prominent artists of his generation. His father, Robert Weir (an art instructor at the United States Military Academy), provided his early training, and J. Alden initially began painting in an established academic style, refined by later study in New York and Paris. Unlike many of his peers, Weir enjoyed notable success as a painter and teacher before he ever began to work in an impressionist manner. He concentrated on portraits, figural works, and still lifes, and these works exhibit rigorous structure, dramatic use of dark and light, and bold handling of pigment.
As Weir became increasingly interested in landscape painting after the purchase of his farm in Branchville, Connecticut (now a National Historic Site), he began to work in a more impressionistic mode, encouraged by his experiments in pastel and watercolor, as well as by his friendships with John Twachtman and Theodore Robinson. Weir gradually adopted the looser brushwork of the impressionist style, altering perspective and flattening forms while choosing to work in a softer color palette than that favored by the more radical practitioners of impressionism. His landscape works include scenes of the western Connecticut hills; Cornish, New Hampshire; the factory landscapes of central Connecticut; and the environs of Cos Cob, Connecticut, where he taught a summer painting class.