Combine rhubarb, sugar, and 1/4 cup water in heavy large saucepan over medium heat. Bring to simmer, stirring occasionally, until sugar dissolves, about 3 minutes. Simmer gently until rhubarb is tender but not falling apart, stirring occasionally, about 7 minutes. Remove from heat. Stir in strawberries. Transfer to bowl and stir in mint. Chill until cold, about 1 hour.
