Although the contest had been much anticipated by boxing fans, it never exploded into life the way many had hoped.
Alexander attempted to control the contest from a distance, circling Bradley, 27-0 (11 KOs), and firing fast jabs and southpaw left hands.
A Bradley right rocked Alexander in the third round, as did several others in the ninth. Bradley's flurries of hard punches saw him build a lead on all three judges' scorecards when the 10th round began.
The newly unified champion was full of praise for his beaten opponent.
"Devon Alexander is a great fighter," said Bradley. "I tip my hat to him. He's going to be a world champion in the future."
(Writing by Kieran Mulvaney in Washington, D.C.; Editing by Peter Rutherford)