When he was 15, he began caddying at Cog Hill Golf & Country Club, founded by the Coghill brothers in 1927 at Lemont, Illinois, 28 miles (45 km) to the southwest of Chicago.

At the age of 17, he became a professional golfer and joined the PGA Tour for a while, then returned to Cog Hill where he took various jobs.

At the 1934 Chicago World's Fair, Jemsek won a trophy for driving a golf ball from the sky-ride tower, 168 feet (51 m) high, into Lake Michigan for a distance of 501 yards (458 m).

In 1947 St. Andrews hosted a U.S. Open qualifier, the first public course to do so, and was the first to offer United States Golf Association (USGA) handicaps for its regular players.

Jemsek employed the champion woman golfer Patty Berg as head professional for almost fifty years.

After John and Bert Coghill had died, in 1951 the remaining brother Marty offered the course to Jemsek.

Jemsek ruled that he could not be admitted to the club house, since if he went in for lunch his drinking would destroy the rest of the day's work.

In 1988, Jemsek was the first public golfer and PGA professional to be nominated a member of the USGA Executive Committee.