Selig will be stepping down shortly after the conclusion of the 2014 season, where one of his final acts will hopefully be handing the Milwaukee Brewers the World Series trophy. Selig had lobbied hard for Manfred to win the role as Manfred has often been described as the commish's right-hand man.
Manfred has been the COO of MLB since last September. Prior to that, he had been Executive Vice President of Economics and League Affairs, a role he had held since 1998. He first got a break in baseball by acting as legal council to the owners during the 1994 player strike.
Manfred also led MLB's investigation that led to Ryan Braun, Alex Rodriguez and over 10 others to be suspended. However, Manfred had come under some fire for the way the league went about their investigation, particularly with regards to how Rodriguez's suspension and appeal turned into a circus.
Red Sox chairman Tom Werner had been the other candidate for commissioner. MLB vice president for business Tim Brosnan had also been a candidate, but dropped his bid early on Thursday. Brewers owner Mark Attanasio had interviewed for the position, but never seemed like a serious candidate.