Complex sentence: Mallory won the singles title at the U.S. Championships a record eight times (1915-1922 and 1926 at age 42), and in 15 years at the tournament, her worst finish was a quarterfinal loss in 1927 at age 43.
Split into simpler sentences: Mallory won the singles title at the U.S. Championships a record eight times in fifteen attempts, with the last of her titles occurring at age 42 in 1926. Her worst finish there was a quarterfinal loss in 1927 at age 43.