Manager Jim Riggleman announced Saturday that the right-handed Mock got the nod over lefty Scott Olsen, who was optioned to Triple-A Syracuse.
Mock learned of his fate, as he did a year ago, at the annual exhibition game at Nationals Park that takes place on the weekend the regular season starts. In 2009, he figured he was here to stay, having made the team as a member of the bullpen.
"Last year, I found out between BP and the game that I was going down, which I didn't expect," Mock said, "so [this year] I was very prepared."
Mock began the spring projected as the No. 3 starter, but he had some struggles early and ended up on the bubble along with Olsen, who returned after shoulder surgery ended his season early last July.
The stats for Mock and Olsen, both 26, were similar this spring. Each allowed 12 earned runs in five starts, although Mock allowed 10 fewer hits in roughly the same number of innings.
"You never want to be so flip about it to say that you could flip a coin as to which one you took," Riggleman said, "but it wavered day to day as to which guy we were going to take."
Riggleman said Olsen is "just not quite there yet in terms of being fully 100 percent," even though Olsen had the better final spring outing. Mock allowed six runs on seven hits in 4 2/3 innings against the Florida Marlins on Tuesday, while Olsen gave up one run on five hits over five innings and struck out seven against the Boston Red Sox on Friday.
"The overall body of work -- we couldn't really just put it all on that one outing," Riggleman said.
Mock's first regular-season start is scheduled for April 9 against the New York Mets. His hold on the spot could be tenuous, with last year's No. 1 overall draft pick Stephen Strasburg expected to move into the rotation in June.
The rest of the rotation consists of John Lannan, Jason Marquis, Craig Stammen and Livan Hernandez. Hernandez remains under a minor league contract and will not be added to the major league roster until his first start April 11. That allows the Nationals to start the season with eight relievers.