If Mulder completes his comeback, he will return to the big leagues after a six year gap. I have come up with a series of pitchers who have had similar gaps in pitching in the big leagues. All have been for different reasons. I did this list off the top of my head, so if there are any other notable gaps among MLB pitchers, please feel free to let me know, either by comment, e-mail or social media. Of course, the pitcher who probably has the longest gap pitching in a big league game was the great Satchel Paige. Making his MLB debut at the age of 41 was something that should never had have to happened. Paige was one of the best to ever throw the ball and should have had 15-20 years of MLB experience by the time he first pitched in 1948. He would pitch in 1949, as well as 1951-1953, with '53 as the last season he figured to pitch on a MLB mound.
12 years later, Paige, at age 58, was given the ball to make a start for the Kansas City Athletics. He would throw 3 innings, giving up just a hit, walking no one and striking out a batter. Jim Palmer would attempt a comeback in spring training of 1991, pitching for the Baltimore Orioles, the team he had just got be inducted in the Baseball Hall of Fame just two years ago. Palmer had received 92.6% of the BBWAA's votes in his first year on the ballot. The 268 games winner with three World Series wins and 8 20 win seasons was unfortunately unable to complete his comeback, which would have been unprecedented for a current Hall of Famer.
It is difficult to talk about pitchers with a lot of time between MLB appearances without bringing up talking about former Cincinnati Reds RHP Joe Nuxhall. Joe, of course, made his MLB debut at the age of 15, the youngest MLB player in history in 1944. He will forever be remembered for giving up 5 runs in 2/3 of an inning. After that, he spent several seasons in the minor leagues before returning in 1952 at age 23. He would spend the next 15 seasons in the big leagues, mostly with the Reds, making the NL All Star team in 1955 and 1956. After his career was over, Joe became a longtime broadcaster of the team.
The next longest gap between MLB pitching appearances was the 7 seasons between appearances that Chuck Hartenstein had. Hartenstein, a guest on my johnpielli.com website (http://www.johnpielli.com/john-piellis-pbs-interviews.html). Chuck had a career that spanned from 1966-1970, mostly for the Chicago Cubs. He would be back in the minors by 1971, but some persistency and adjustments got him some accolades in the minors. One of his managers was Roy Hartsfield, who had just gotten the MLB job with the expansion Toronto Blue Jays. Hartenstein was taken over with Hartsfield and would make 13 appearances for the Jays in 1977, going 0-2, 6.59 in 27 1.3 IP.
Next was the return of former Cincinnati Reds top pitcher Jose Rijo. Rijo pitched for 12 seasons in the big leagues, mostly for the Reds, winning the World Series with the team in 1990. By 1995, his arm was shot. His season ended prematurely, as he was long gone by the time the team swept the Dodgers in the NLDS and got swept by the Braves in the NLCS. After multiple operations, Rijo was back. He returned to the Reds in the 2001 season, pitching to a 2.13 ERA in 13 relief appearances. The next season, Rijo pitched in 31 games, going 5-4, 5.14, as he also made 7 starts at age 37.
Another interesting one is the path of current San Francisco Giants RHP Ryan Vogelsong. Vogelsong had some struggles during his time in Pittsburgh, as he pitched with them in 2001-2002 and 2003-2006. He never established himself and by the end of 2006, was out of baseball. He attempted his comeback in 2010, pitching in AAA for the Phillies and Angels. Though he never made it to the majors, he would sign a minor league deal with the San Francisco Giants for the 2011 season. He earned himself a callup and would go 13-7, 2.71 in 30 games, 28 starts. He struck out 139 batters in 179 2/3 IP. He went 14-9, 3.37 in 2012 for the Giants as the team won its second WS in three seasons. He battled some injury issues in 2013, but is set to return to the Giants rotation in 2014, as he signed a 1 year, $5 million contract for the season.
Dave Stieb had a very good career for the Toronto Blue Jays from 1979-1992, before making 4 starts for the Chicago White Sox in 1993. It was assumed that his career was finished as Stieb had struggled pitching in the minors for the White Sox and Kansas City Royals. But he signed a minor league deal with the Blue Jays in 1998 and pitched very well in A+ and AAA. He made his return to the big leagues by going 1-2, 4.83 in 19 games, 3 starts for the Blue Jays.
Based on my research, I could not find a pitcher that had a gap of 5 years or more based on service time in either World Wars, or Korea. Of course, Steve Howe had a 4 year gap after having multiple suspensions related to the use of drugs.