He was administered the oath of office by President Pranab Mukherjee at Rashtrapati Bhavan.
64-year-old Mathur succeeds Sushma Singh, who demitted office yesterday, as the sixth chief of the transparency watchdog.
His name was recommended by a three-member selection panel headed by outgoing Prime Minister Manmohan Singh. "My priority would be to reduce pendency of appeals and complaints, and to strengthen RTI apparatus.
One of the priorities of the Commission would be to ensure adherence of Section 4 of RTI Act (which mandates public authorities to disclose information related to their working in public domain) by various government departments," Mathur told PTI.
There are about 14,000 appeals and complaints pending with the CIC, he said. He will have a tenure till August 23, when he attains 65 years of age.
According to rules, CIC is appointed either for a maximum of five-year term or till 65 years of age, whichever is earlier.
Mathur, a retired IPS officer of Uttar Pradesh cadre, was Director of Intelligence Bureau for two-year term from December 31, 2008 and was instrumental in revamping the intelligence set up and raising the quality of intelligence gathering and sharing.
He also galvanised the IB machinery and provided leadership to the organisation to face challenges, according to Mathur's official biodata. He is recipient of the Indian Police Medal for Meritorious Service and the President's Police Medal for Distinguished Service.
Mathur had in March 2012 joined as Information Commissioner in the CIC. His predecessor Sushma Singh had joined as CIC on December 13, last year.
Deepak Sandhu, Satyananda Mishra, A N Tiwari and Wajahat Habibullah have also served as CICs.
There are seven Information Commissioners-- Vijai Sharma, Basant Seth, Yashovardhan Azad, Sharat Sabharwal, Manjula Prasher, M A Khan Yusufi and Prof Madabhushanam Sridhar Acharyulu-- against the Commission's strength of 10.
The CIC is mandated to resolve appeals and complaints filed by information seekers, under the Right to Information (RTI) Act, against government departments or public authorities.