Of American-English origin, the phrase Mr Nice Guy designates a pleasant, selfless, thoughtful person.

All the earliest occurrences of Mr Nice Guy that I have found:
– are used of the U.S. singer, film actor and television-show host Perry Como (Pierino Ronald Como – 1912-2001);
– are from articles on radio and television by Jack O’Brian, of the International News Service, published:

1-: In several newspapers in August and September 1952—for example in The Tipton Daily Tribune (Tipton, Indiana) of Saturday 30^th August:

New York—(INS)—[…] Reviewlets: “The Perry Como Show” returned this week to CBS-TV. There isn’t much to add to last year’s glowing reports about “Mr. Nice Guy.” His manner is simple, sincere, easy and his singing as good as always.

2-: In several newspapers in September 1952—for example in The Daily Herald (Jasper, Indiana) of Wednesday the 10^th:

New York, Sept. 10—(INS)—Mr. Nice Guy: That’s what they call Perry Como around radio-TV, and we don’t know a soul who disagrees.

3-: In several newspapers in September 1952—for example in The Daily Herald (Jasper, Indiana) of Thursday the 18^th:

New York, Sept. 17—(INS)—[…] Mr. Nice Guy . . . That would be of course Perry Como, whose public performances engender lots of respect over at these reactions. An even nicer facet of the baritone gent’s makeup came to our attention yesterday when we learned, virtually over Perry’s dead body, that he and his wife last Saturday morning were given one of the high honors of the Catholic Church; Perry and Roselle were made Grand Knights of the Holy Sepulchre, in a ceremony presided over by Cardinal Spellman.

4-: In several newspapers in November 1952—for example in the Columbus Evening Dispatch (Columbus, Ohio) of Wednesday the 19^th:

New York, Nov. 19—(INS)—[…] Perry Como’s recording of “One Little Candle” has been chosen by the 3000 tuberculosis ass’ns as the 1952 Christmas Seal song. Says Mr. Nice Guy: “Nothing has been so inspiring as this wonderful chance to help the fight against tuberculosis.”

Of American-English origin, the phrase no more Mr Nice Guy is used to express that one has decided to stop being considerate of others and instead act exclusively in one’s own self-interest.

The earliest occurrence of no more Mr Nice Guy that I have found is from On Broadway, by the U.S. columnist Walter Winchell (1897-1972), published in several newspapers in September 1960—for example in the Daily Record (Boston, Massachusetts) of Thursday the 8^th:

Hitler ¹ Vignette: The gag deals with Adolf’s ordeal in the bunker just before he shot himself dead . . . The Yanks and the Russians devastated Berlin and Germany was licked . . . One of his aides dashed in and told Hitler all was lost. “The Russians,” he reported, “are advancing on us and the American bombers have set the city on fire. There is no escape!” . . . Hitler hammered on the table and shouted: “That does it! From now on—no more Mr. Nice-Guy!”

¹ The Austrian-born Nazi leader Adolf Hitler (1889-1945) was the Chancellor of Germany from 1933 to 1945.

The second-earliest occurrence of no more Mr Nice Guy that I have found is from The Jersey Journal and Jersey Observer (Jersey City, New Jersey) of Wednesday 13^th March 1963:

K Decides: No More Mr. Nice Guy
By K. C. Thaler

London (UPI)—Premier Nikita Khrushchev ², under mounting fire from Red China, apparently is out to set his Marxist record straight.
The Soviet leader’s latest strategy emerged in a series of recent moves, all pointing strongly to a calculated tightening of the Kremlin’s line over a wide field of Soviet activity.
Whether in readiness for some form of truce with Peking or for a public showdown with his ally, Khrushchev is to all appearances trying to prove that he is a tough Communist.

² The Soviet statesman Nikita Sergeevich Khrushchev (1894-1971) was the Premier of the USSR from 1958 to 1964 and the First Secretary of the Communist Party of the USSR from 1953 to 1964.