After a contract holdout in 1949, Hansen's bullpen workload increased in 1950, leading to an elbow injury and the Giants sending him to the Phillies in the Rule 5 draft.

[4] After graduating that year, Hansen spurned football scholarship offers from "a number of schools, including Georgia Tech"[3] to sign an amateur free agent contract worth $75 ($1,320 today) per month with the National League's New York Giants.

[3][5] The Giants assigned Hansen to their Appalachian League affiliate, the Bristol Twins, where, at age 18, he posted a "sensational [win–loss] record"[4] of 12–3 in 16 games started.

Managed by Hall of Fame catcher Gabby Hartnett, Hansen posted an 8–4 record, compiling a 1.89 earned run average (ERA) in 11 starts and 4 relief appearances.

[14] Sporting a 3–2 record, Hansen earned his first career save against the Phillies on August 6, pitching two scoreless innings in relief of Harry Feldman and striking out two.

[5] He lost his third decision of the year on August 19, allowing five runs and walking six batters[16] in the Giants' 12th consecutive loss out of a season-long 13-game streak.

[17] For his rookie season, Hansen finished with a 3–3 record and a 6.49 ERA in 52+2⁄3 innings pitched; he appeared in 23 games (4 starts) and struck out 15 batters while walking 32 at age 19.

[19] Hansen started New York's third game of the season, pitching against the Boston Braves; he struck out three and allowed one run through seven innings, earning the victory.

[44] At the beginning of August, Hansen injured his shoulder[45] and was optioned to Jersey City in favor of Sal Maglie,[46] where he appeared in five games (1–3, 7.31 ERA).

[57] On July 28, he turned in a hitless, scoreless performance against Cincinnati, working one inning in a 5–0 shutout,[58] and received his first decision—a loss—against the Reds on the final day of that month in an 8–7 contest.

[61] Closer Ken Trinkle, who relieved Hansen, took the loss after a three-run home run by Tommy Holmes in the ninth inning as the Giants lost, 7–5.

Hansen walked Arky Vaughan, the first batter in the eighth inning, before leaving the game, and Cookie Lavagetto singled with the bases loaded to drive in the winning runs for the Dodgers;[66] the Giants fell, 3–1.

He pitched a complete game, allowing three runs (two earned) on four hits; the 5–3 victory was his only win of the 1947 season, defeating Philadelphia starter Ken Heintzelman.

[76] On September 3, Hansen was pressed into service as a starter for the first time in two months of play, and he delivered a quality start for New York, allowing three runs in eight innings pitched—his third victory and first complete game of the season.

[92] He was rewarded with another start on September 7, and pitched his second consecutive complete game to earn a fourth win, this time scattering ten hits in nine innings against the Phillies, striking out three batters and allowing only a home run to Bert Haas.

[100] In the next game against the Dodgers, however, he recorded his first victory of the year behind a pinch-hit inside-the-park grand slam from Pete Milne,[101][102] pitching a scoreless seventh inning and striking out two.

[108] Hansen earned his second defeat of the season on June 12, when he allowed a single game-winning run to the Pirates in 5+2⁄3 innings of work; he walked three batters and struck out five in the contest.

[119] In his first September game, Hansen earned his final decision of the season when he defeated Pittsburgh, 9–5, pitching one scoreless inning in relief of Hank Behrman.

[128] Beginning on May 25, Hansen pitched in four consecutive games to close out his month, allowing 1 run in 2+1⁄3 innings in the first contest and throwing a scoreless frame in the second.

On the month's first day, Hansen pitched in both games of a doubleheader against Cincinnati, throwing a perfect 1⁄3 of an inning in the first half, and three scoreless in the second contest.

The Braves, Philadelphia's opponents in that contest, won the game, 9–7, and Hansen was saddled with his second blown save in as many appearances and his first loss of the season.

[171] In his final June game, he allowed three runs, but none were earned due to a throwing error by third baseman Willie "Puddin' Head" Jones.

[172] At the end of June and beginning of July, closer Konstanty blew two saves, one of which resulted in a 4–3 loss to Brooklyn,[173] so Hansen assumed some of the closing duties for the season's fourth month.

[162] In August, Hansen pitched 4+1⁄3 innings in the team's first contest, allowing two runs to the Cardinals on four hits and losing his third game of the season.

[179] Hansen rested until August 22, when he relieved Curt Simmons against the Cincinnati Reds; he pitched 3+2⁄3 innings, allowing two runs on three hits, walking two, and absorbing a fourth loss.

[187] Hansen's first appearance in May came against the newly relocated Milwaukee Braves, late of Boston, when he allowed a walk and two hits, one a home run, in a single inning.

[196] Waiting ten days until his next appearance, Hansen took the mound for two scoreless innings against St. Louis,[197] and earned a hold on June 30 against Brooklyn after Konstanty blew a save.

[198] Two days later, he pitched five scoreless innings within both halves of a doubleheader against his former team, the Giants; he walked one batter in each game and struck out three hitters in the second.

[206] After being voted outstanding player of the week for his short tenure, Hansen voluntarily retired himself a second time after Pittsburgh Pirates president Branch Rickey refused to pay him $750 ($8,510 today) that he was owed.

[4][207] He had dated his second wife, Joy, when he was playing for the Giants in his early career,[207] and they were described as "recently wedded" when interviewed by author Wes Singletary for his 2006 book.