Lawyer Alan Watenmaker told a hearing on Monday that Mitchell, 71, may leave hospital soon but is unable to confer with doctors about her medical care.
Leslie Morris, Mitchell's friend for more than 40 years, was appointed guardian by judge David S Cunningham.
It is still not clear what the singer-songwriter is being treated for.
Last week her website said she was "alert and... has her full senses", revealing that she is "not in a coma".
Mitchell has been in intensive care at a Los Angeles hospital since 31 March for undisclosed reasons and no further information about her condition was discussed during Monday's hearing.
The eight-time Grammy winner has no relatives who can serve as her conservator.
After filing papers last week, Morris will now be able to confer with doctors and make decisions about her treatment and lifestyle when she leaves hospital.
Lawyer Rebecca Thyne, appointed to represent Mitchell's interests, agreed that Morris should receive emergency conservatorship authority, calling it "very necessary".
Morris, however, will not have any control over Mitchell's finances.
The Canadian-born singer, known for such hits as Big Yellow Taxi and Chelsea Morning, revealed last year she had a rare skin condition that prevented her from performing.
A judge has put a friend of singer Joni Mitchell in charge of her medical decisions, after the musician entered intensive care last month.