Mary Ann Eagin is the main villainess from "Murder at the Finish Line," a fifth season episode of Diagnosis Murder.
Mary Ann Eagin is the villainous owner of Eagin Racing, but she was revealed to be having financial problems. She used the money raised for her team to cover her losses, but she oversold her shares to various investors. One of her racers, Matt Thomas, started winning numerous races, which made a lot of money for Eagin Racing. But knowing that the investors would want a return on her money, Mary Ann became a villainess hellbent on stopping Matt's winning streak. She poisoned Matt hours before his race, and to make sure he would be killed, she sabotaged the engine in his car, leading to his crash.
Once the team was able to get a new engine working, the evil Mary Ann snuck into the garage later that evening and sabotaged it, pouring an explosive into the engine. Mary Ann was revealed as the villainess when she ducked for cover while the engine was being tested, something Mark and Steve Sloan counted on. Mary Ann was arrested for her crimes, and she was left incredulous over how Matt was winning so much.