ASGN (ASGN) S-8 Registration of securities for employees

Exhibit 23.1

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Registration Statement on Form S-8 of our reports dated February 28, 2018, relating to the consolidated financial statements and financial statement schedule of On Assignment, Inc. and subsidiaries (the “Company”), and the effectiveness of the Company’s internal control over financial reporting, appearing in the Annual Report on Form 10-K of the Company for the year ended December 31, 2017.

/s/ DELOITTE & TOUCHE LLP

Los Angeles, California

March 27, 2018