“Don’t get mad—get even!” is often attributed to Joseph P. Kennedy, Sr. (1888-1969), the father of John F. Kennedy and Robert F. Kennedy. In November 1966, Life magazine called this “a maxim among Old Tadcasters—Boston’s barroom politicians.”
 
The origins of the saying remain unknown. “Don’t get mad—get even” is cited in a 1956 “Teen Scene” article about “Favorite Expressions.” In 1979, “Dirksen’s Three Laws of Politics” (after Illinois politician Everett Dirksen, 1896-1969) were written as: “1. Get elected. 2. Get reelected. 3. Don’t get mad, get even.” Political consultant James Carville said in 1992: “Don’t get mad. Don’t get even. Just get elected. Then get even.’’
 
“Don’t get mad—get everything!” was said by Ivana Trump in a film cameo appearance in The First Wives Club (1996).
     
 
The Free Dictionary
Don’t get mad, get even.
something that you say in order to tell someone not to be angry when someone has upset them, but to do something that will upset them as much This is my advice to wives whose husbands have left them for a younger woman - don’t get mad, get even! ‘t get mad, get even.
   
Google Books
The Yale Book of Quotations
Edited by Fred R. Shapiro
New Haven, CT: Yale University Press
2006
Pg. 423:
Joseph P. Kennedy
U.S. businessman and politician, 1888-1969
“Don’t get mad, get even.”
Quoted in Ben Bradlee, Conversations with Kennedy (1975).  An earlier occurrence appeared in the Chicago Tribune, 21 Feb. 1967: “The motto of the Irish Mafia which Bobby [Kennedy] inherited has always been, ‘Don’t get mad—get even,’ a slogan which predates the Kennedys in Massachusetts politics.”
     
14 June 1956, Newport (RI) Daily News, “Teen Scene,” pg. 15, col. 1:
FAVORITE EXPRESSIONS:
(...)
Cathy Janes—“Don’t get mad—get even.”
 
Google Books
Are you my mother?
By Philip D. Eastman
New York, NY: Beginner Books; distributed by Random House
1960
Pg. 49:
He thought of all the time he had wasted in the office wallowing in depression and said, “Don’t get mad. Get even.”
 
Google Books
The one-eyed man
By Larry L. King
New York, NY: New American Library
1966
Pg. 107:
I’m not mad, for to get mad would discredit my philosophy. I got a motto I owe boatloads of allegiance. And it goes: ‘Don’t get mad…get even!
   
Google News Archive
16 July 1966, Ottawa (Ontario) Citizen, “Tough old Governor got in history’s way” by Winston Mills, pg. 24, col. 4:
The One-Eyed Man, by Larry King
General Publishing Co, Ltd., 309 pp; $6.95
As Southern as the mocking bird is this story of Cullie Blanton, coarse governor of an unnamed state in the Deep South, narrated by Jim Clayton, his long-time aide and confidante.
(...)
“Don’t get mad during political campaigns,” he told his admirers. “They don’t do no good. But get even!”
 
Google Books
18 November 1966, Life magazine, pg. 135, col. 1:
There is a maxim among Old Tadcasters—Boston’s barroom politicians: “Don’t get mad. Get even.” The primitive instincts in Bobby (Kennedy—ed.) still do battle with his new urbanity.
 
21 February 1967, Chicago (IL) Tribune, “Can L.B.J. lick the Irish Mafia?”, pg. 20:
The motto of the Irish Mafia which Bobby inherited has always been, “Don’t get mad—get even,” a slogan which predates the Kennedys in Massachusetts politics.
 
Google News Archive
5 May 1967, Times-News (Hendersonville, NC), “Lighter Side of Life” by Dick West, pg. 2, col. 7
The besi you can hope for is revenge. Or, as they say in Boston political circles, don’t get mad, just get even.
 
7 January 1971, Los Angeles (CA) Times, “Newsmakers,” pg. C2:
“There is an old Scotch-Irish saying, ‘Don’t get mad, just get even.’ I have a score to settle with Mr. Nixon, Agnew and Sen. Howard Baker.” Along with that announcement of vendetta Albert Gore disclosed he would “get even” by writing a book on the office of the President and election manuevering. Gore, a Tennessee Democrat, lost his Senate seat to Baker, a Republican, in the November election after 32 years in Congress.
   
OCLC WorldCat record
Alias Smith and Jones. Don’t get mad, get even
Publisher: [1972-07-27]
Edition/Format: Film : English : [Summer rerun of program originally broadcast on Feb. 17, 1972]
   
Time magazine
AMERICANA: Beasts and Burdens
Monday, Feb. 26, 1979
(...)
Dirksen’s Three Laws of Politics. 1. Get elected. 2. Get reelected. 3. Don’t get mad, get even.−The late Senator Everett Dirksen
 
1 December 1992, USA Today, “Campaign Odd Couple,” pg. A4:
“Don’t get mad. Don’t get even. Just get elected. Then get even.’’
(James Carville—ed.)