We present leakage-free valves and an efficient fixed-volume mixer fabricated using a four-layer soft lithography technique in polydimethylsiloxane (PDMS). Mixing is achieved with a 95% efficiency in a few seconds by creating circular flow patterns inside a simple rectangular chamber. Pressurized air is used to actuate control chambers for active valving and mixing. In our device, neither the mold nor the microfluidic channel requires complex designs.