Born in Davenport, Iowa, Weir started swimming while the family lived in Apple Valley, Minnesota.

Weir also earned a silver medal by swimming in the preliminary heats of the 4×100-meter medley relay and helping her team qualify for the final.

At the 2006 ConocoPhillips USA Swimming National Championship, Weir broke the American record in the 100-meter freestyle with a time of 53.58.

At the 2012 United States Olympic Trials in Omaha, Nebraska, Weir made the U.S. Olympic team for the second time by finishing fifth in the 100-meter freestyle with a time of 54.41,[6] which qualified her to swim in the 4×100-meter freestyle as a member of the U.S. relay team.

At the 2012 Summer Olympics in London, she earned a bronze medal as a member of the U.S. team in the women's 4×100-meter freestyle relay.