My partner and I are US citizens and wish to relocate to the greater Toronto area. Do we need to secure employment before applying for a work permit? We are both social workers and should qualify for skilled worker status.

That will depend on the time frame within which you are looking to relocate: more than 1-2 years, then possibly no (depending on your ability to meet the 67 point threshold of the Skilled Worker program); less than 6-12 months, then probably yes (in order to be able to seek a work permit).