Donaldson joined the Dallas Mavericks in 1985. He joked with teammates that leaving the lowly, dysfunctional Clippers for the Mavericks was like dying and going to Heaven. He had his finest years while playing for the Mavericks, providing rebounding and shot-blocking to complement Dallas' star-studded line-up, which included Mark Aguirre, Rolando Blackman, Roy Tarpley, Derek Harper, Sam Perkins, and Brad Davis. Donaldson himself earned a spot on the 1988 All-Star Team during a season in which the Mavericks reached the Western Conference Finals before losing to the Los Angeles Lakers. The NY Daily News named him the worst All-Star player ever after a fans voting. Sadly, things fell apart for the Mavericks generally in the late 1980s and early 1990s as their core group was either traded away (like Aguirre) or squandered vast potential via personal problems (like Tarpley) and Donaldson became the target for many fans and even his teammates for the franchise's woes, making the end of his otherwise hugely successful tenure in Dallas inevitable.