You can also find biographies of particular mathematicians by doing keyword searches on their names in DELCAT Discovery, the University Libraries' catalog.
The Library of Congress Subject Headings, used in DELCAT Discovery, has several terms that enable you to identify biographical sources. The most relevant of these are: biography, autobiography, autobiographies, bio-bibliography, interviews, obituaries, and personal narratives. Specify one of these words as a subject keyword along with the name of biographee, e.g. su: mary somerville biography.
Compilations of biographical sketches are often called "dictionaries” (e.g., Scientists -- Biography -- Dictionaries).
Searching Google or another search engine using a particular mathematician's name, for example: Hypatia, Euler or Ramanujan, will help you find web sites with biographical information specific to that person. Here are some general biographical web sites for mathematicians.