After starring for the Cal Poly Mustangs, the Milwaukee Brewers selected Haniger in the supplemental section of the first round of the 2012 MLB draft.

Haniger attended Archbishop Mitty High School in San Jose, California, part of the West Catholic Athletic League.

[5] He was named the 2010 Big West Conference Freshman of the Year following his debut season with the Mustangs, in which he batted .326.

[7][8] He once again played summer collegiate ball, this time with the Green Bay Bullfrogs of the Northwoods League.

[3][12] The Brewers received that pick as compensation for the loss of free agent slugger Prince Fielder the previous offseason.

[13] Haniger debuted professionally that summer in 14 games for the Wisconsin Timber Rattlers of the Class A Midwest League.

[14] His season ended in early July after he suffered a partial tear of his posterior cruciate ligament in his right knee, which he injured on a play at the plate.

[15][16][17] Haniger began the 2013 season rated as the Brewers' 10th-best prospect and best outfield arm by Baseball America.

[15] He returned to Wisconsin to start the season, before being promoted to the Brevard County Manatees of the Class A-Advanced Florida State League in late May.

[15] Failing to make the team's 25-man roster, Haniger was assigned to the Huntsville Stars of the Double-A Southern League to begin the season.

[23][24][25] On July 31, 2014, the Brewers traded Haniger and Anthony Banda to the Arizona Diamondbacks for Gerardo Parra.

Though he batted .281/.351/.379 in 55 games for Mobile, the Diamondbacks demoted him to the Visalia Rawhide of the Class A-Advanced California League in late June so that he could play more frequently.

[36] After the season, he was named the Diamondbacks Minor League Player of the Year after batting .321 with 25 home runs and 12 steals for Reno and Mobile.

[15] On November 23, 2016, the Diamondbacks traded Haniger, Jean Segura, and Zac Curtis to the Seattle Mariners for Ketel Marte and Taijuan Walker.

[38] Haniger was rated the fifth-best prospect in the Mariners' farm system by Baseball America heading into the 2017 season.

[40] He went on the 10-day disabled list (DL) with a strained oblique muscle on April 25, returning to the Mariners on June 11.

[42] On July 29, Haniger was hit in the face by a 95-mile-per-hour (153 km/h) fastball from Jacob deGrom of the New York Mets,[43] sending him back to the DL.

He came back to the Mariners on August 19 and hit his first career grand slam, homering off Jake Odorizzi of the Tampa Bay Rays.

[45] Coming off a productive yet injury-shortened rookie campaign, Haniger hit .272/.358/.488 with 18 home runs and 67 RBIs in 94 games before the All-Star break.

He reached base at a high frequency, ranking 11th in on-base percentage and tied for 12th in walks in the AL.[49][50] In addition, he displayed excellent defense in the outfield.

[54] Following the departure of teammates including Robinson Cano, Nelson Cruz, James Paxton and Edwin Diaz in the offseason, Haniger found himself as the new leader of the rebuilding Mariners team.

[42][56] After a slow start, in which he hit .220/.314/.463 with 15 homers in 63 games,[36] Haniger was placed on the injured list (IL) with a ruptured testicle after fouling off a fastball into his groin area on June 6.

[36] On January 5, 2024, the Giants traded Haniger, pitcher Anthony DeSclafani, and cash considerations to the Mariners for injured starter Robbie Ray.

[2][83] Haniger was college roommates with his childhood best friend and high school teammate Elliott Stewart, who later became a coach at Cal Poly.