Mueller checked into an undisclosed in-patient treatment centre in December following her arrest on suspicion of felony cocaine possession in Aspen, Colorado.
A representative for Mueller has now disclosed that the 34-year-old has completed her in-patient rehab stay and has returned to LA for outpatient treatment.
Her rep said in a statement: "After successfully completing an intensive three-month drug and alcohol treatment programme, Brooke returned to Los Angeles yesterday and has immediately begun an aggressive outpatient programme that includes a sober companion.
"Brooke is feeling better than ever before, and is committed to continuing treatment for as long as necessary to maintain her total and complete sobriety."
Mueller previously completed a stint in"extreme" plant therapy in Mexico last summer.
It was reported last month that Mueller will not be granted a plea deal in her cocaine possession case.