He has previously played in MLB for the Kansas City Royals, San Francisco Giants, Milwaukee Brewers, and Cincinnati Reds.

He pitched in 2013 with the Idaho Falls Chukars, posted a 2–6 record and 7.39 ERA in 13 starts, and was 8th in the Pioneer League with 55 strikeouts.

[1][9] Junis pitched for the Wilmington Blue Rocks (with whom he was 2nd in the Carolina League with 123 strikeouts, and third in WHIP (1.12)) and Northwest Arkansas Naturals in 2015, where he compiled a combined 5–12 record and 3.78 ERA in 27 starts.

[1][13] In 27 combined starts between Northwest Arkansas and Omaha he was 10–10 with a 4.05 ERA, and 143 strikeouts and 34 walks in 149 innings, as he generally threw a fastball in the low-90s and occasionally slightly higher, a change-up, and a curveball.

[16] After his debut, he was recalled and optioned a number of times before he was called up to Kansas City for the remainder of the season on August 14.

[1] Junis made 16 appearances (six starts) for the Royals in 2021, posting a 5.26 ERA with 41 strikeouts in 39.1 innings of work, and was on the injured list twice.

[25] In 15+1⁄3 innings split between the Triple–A Sacramento River Cats and rookie–level Arizona Complex League Giants, Junis accumulated a 7.04 ERA with 13 strikeouts.

[25] On January 13, 2023, Junis agreed to a one-year, $2.8 million contract with the Giants, avoiding salary arbitration.

[27] On February 7, 2024, Junis signed a one-year $7 million contract with the Milwaukee Brewers that contained a mutual option for a second year.

On July 30, 2024, the Brewers traded Junis, Joey Wiemer, and cash considerations to the Cincinnati Reds in exchange for Frankie Montas.