Sandeep Unnikrishnan received a substantive promotion to captain on 12 June 2003, followed by promotion to major on 13 June 2005. During the 'Ghatak course' (at the Commando Wing (Infantry School), Belgaum), which is considered as the most difficult course of the Indian Army, Sandeep topped the course twice, earning an "Instructor Grading" and commendation. He was also trained in High Altitude Warfare school, Gulmarg. After serving in different locations including Siachen, Jammu and Kashmir, Gujarat (during 2002 Gujarat Riots), Hyderabad and Rajasthan, he was selected to join the National Security Guards. On completion of training, he was assigned as the training officer of the 51 Special Action Group (51 SAG) of NSG, on January 2007 and also participated in various operations of the NSG.