Mackowiak will earn $1.5 million, while Harris will get $800,000.
"With the additions of Mackowiak and Harris, I feel we will have one of the deepest and most balanced benches in the National League," general manager Jim Bowden said in a statement.
Mackowiak is a career .262 hitter and has played all three outfield positions, as well as first, second and third base. Mackowiak hit a combined .278 with six homers and 36 RBIs in 85 games with White Sox and Padres last season. He hit .283 (13-for-46) against left-handed pitching.
Harris appeared in 117 games with the Braves in 2007 and hit .270 with two homers, 32 RBIs and 17 stolen bases.
Experienced at every defensive position except first base, catcher and pitcher, Harris has 147 career starts at second base, 75 in center field, 71 in left field, two at shortstop and one in right field. He also appeared twice defensively at third base for Atlanta in 2007.
In other news, the Nationals agreed to terms on a Minor League deal with left-hander Mike O'Connor, who was non-tendered on Wednesday. O'Connor hasn't pitched in the big leagues since 2006. He missed a good chunk of the '07 season because of an elbow injury. When he returned to action, O'Connor pitched for Double-A Harrisburg and went 3-7 with a 7.07 ERA.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.