After going 10-10 at AA Waterbury in 1969, he got the September call up having to face Hall of Famer Rod Carew in his first appearance.
Hennigan spend some more time in the minors, but would mostly pitch out of the Cleveland bullpen from 1969-1972.
In 1970 he was 6-3 with three saves posting a 4.02 ERA in 42 games. He became Cleveland’s ace reliever in 1971, going 4-3 with 14 saves (8th best in the A.L.) posting a 4.94 ERA in 57 games. He dropped off to six saves, third best on the Indians staff in 1972 behind Steve Mingor (10) & Ed Farmer (7) while posting a 5-3 record.
That November he was traded to the New York Mets as they wanted to booster up their bull pen. In exchange pitchers Brent Strom & Bob Rauch were sent to the Indians. Hennigan was only 27 years old at the time, but his Mets career would turn out to be short lived.
In 1973 Hennigan got the save for the Mets in their third game of the season, helping Jerry Koosman in a 5-4 win over the Cardinals in St. Louis. The next day he saved another in a tight 2-1 Mets win. Henneigan was used often in the early part of the season, but then his next three decisions were losses.
On June 5th in Cincinnati he relieved Tug McGraw in the 10th inning with Mets ahead 5-2. Duffy Dyer had cleared the bases with a three run triple in the top of the inning. McGraw had allowed a run to score & left two men on base as well. Hennigan then gave up a three run walk off HR to catcher Johnny Bench taking the loss.
After making thirty appearances he was be 0-4 with an ERA with four saves posting a 6.23. In July the Mets were still struggling in last place, and having problems in the bull pen.
Hennigan was sent down to AAA Tidewater and spent the rest of the year there, as the Mets went on to win the NL Pennant. He pitched at AAA Tidewater in 1974 as well going 0-3 finishing out his playing career.
He will forever be remembered as a Met by his 1973 Topps baseball card, where his hat was air brushed with a Mets logo. Lifetime in a five season career, he was 17 -14 with 25 saves a 4.26 ERA, 174 strike outs & 193 walks in 120 games pitched.
Retirement: After baseball he returned to Texas, working for the Dept. of Public Safety, as well as the Sheriff's Dep & Police Department where he retired a Sergeant. He was married to his wife Linda Kaye from 1986 to his passing. Hennigan passed away June 17th, 2016 in Texas.