In the
1958 NFL draft he was selected in the 20th round as the 239th pick by the San Francisco 49ers. In the
1958 season that followed, Mertens started all 12 games for the Niners at left cornerback, grabbing two interceptions and recording one fumble recovery. He was named to the
1958 Pro Bowl following the season and was part of the Western Conference's 26–7 victory over the Eastern Conference in that contest. Mertens started at left cornerback in 60 of the 64 games played by the 49ers during his first five years in the league. "I've never played safety before," Mertens told a bay area sports page editor, "but I like it — and I think I'm getting the hang of it." Mertens had high expectations for the 49ers squad in
1963, noting that "We're going to be a lot stronger than anyone thinks. For one thing, we have better spirit, and that will make for a better performance. More important, we're deeper. Injuries killed us last season — we can stand a few injuries this season without being hurt too much." caused Mertens to miss the entire 1963 campaign. he was not the same player and saw himself limited to reserve duty for all 14 games that year. Mertens started 8 games for the team at right cornerback during the year and saw action in 13. It was not until the final game of the season, at home against the
Green Bay Packers, that Mertens was healed up well enough to return to the starting lineup. Mertens felt a sharp pain in his neck, forcing him to the sidelines for the rest of the day. It would be four days before x-rays revealed a cracked vertebrae in Mertens' neck. Several weeks in the hospital in
traction resulted. Twelve weeks later he was freed from the cast and fitted with a lighter brace, finally allowed to move his neck slightly. After 8 years in the NFL, with 7 years of service, the 30-year old Jerry Merten's professional football career was over. ==Life after football==