Callis began her legal career as a prosecutor in the state's attorney offices of Madison County and St. Clair County. Callis was sworn in as an associate judge in 1995 to Illinois's Third Judicial Circuit. At the time of her selection for the court, she was the youngest of 21 candidates. In the same year, she was given the "lowest rating for legal ability among 152 judges in Illinois outside of Chicago" by an Illinois State Bar Association poll. Callis noted that she had not received any complaints about her performance, and the low rating reflected the fact that she had been on the bench only a few months and most lawyers had not appeared before her. According to the St. Louis Post-Dispatch, prior to Callis' appointment there were "months of reports she had the job locked up because of her influential father" and she "was dogged early in her career by allegations that she got to the bench because of her influential father." She was named a circuit judge in 1999, and chief judge in 2006.
As a judge, Callis was involved with the creation of the first Veterans’ Court in Illinois, which received the national 2010 Paul H. Chapman Award. The court is run by veterans and provides services to county veterans.
Callis also created mediation programs aimed at helping homeowners and lenders avoid mortgage foreclosures.
In a 2012 poll of lawyers, Callis was recommended for retention, with 83.64% of those responding saying Callis "meets requirements of office." Three other judges from the 3rd Judicial Circuit received higher scores than Callis. Her lowest score in the judicial evaluations, 82.63%, was in the category "impartiality." Her highest scores were 98.8 in "health" and 92.77 in "sensitivity."