Mahlon Marsh Day (1913–1992) was an American mathematician, who specialized in functional analysis, geometry of linear spaces and amenable semigroups.
In most of his career, between the years 1940–83, he was a professor of mathematics in University of Illinois at Urbana-Champaign.
In June 1983, a conference named "the Geometry of Normed Linear Spaces" was held in honor of Day at the University of Illinois at Urbana-Champaign.
In the preface for this proceedings issue, Day was described as "the first American mathematician to study normed spaces from a geometric standpoint".
His monograph "Normed Linear Spaces" from 1973 is highly cited and considered to be classical in the field.