McDonnell made his senior debut for Cork during their 1969–70 National League campaign. It was a successful campaign for the Rebels as an aggregate 5–21 to 6–16 defeat of New York gave McDonnell his first National Hurling League medal. The subsequent championship campaign saw McDonnell win his first Munster medal as Tipperary were beaten 3–10 to 3–8. Cork subsequently faced Wexford in the All-Ireland decider on 6 September 1970. A record 64-point scoreline and eleven goals were produced in the contest. The day belonged to Eddie O'Brien who scored a hat-trick of goals for Cork from his hand as Cork secured a 6–21 to 5–10 victory. It was McDonnell's first All-Ireland medal. He finished the year by becoming the youngest recipient of the Texaco Hurler of the Year award.