Signed as a minor league free agent by the Los Angeles Dodgers in 1998, Arnold made his Major League Baseball debut with the Dodgers on April 20, 1999 against the Braves, working 2 scoreless innings. He pitched 36 games for the Dodgers in 1999, including 3 starts.
However, he failed to make the Dodgers opening day roster in 2000 and spent the first half of the season primarily with the Albuquerque Dukes. On July 26, 2000 the Dodgers traded him (along with Jorge Piedra to the Chicago Cubs for Ismael Valdes. Arnold pitched in 12 games for the Cubs that season, including making 4 starts.