Jilani reached the US capital seven months after the previous envoy, Sherry Rehman, quit and returned home when the Pakistan People's Party lost the general elections in May, Geo News reported.
This post was vacant since the resignation of Rahman.
Talking to Geo News after joining office Monday, Jilani said he would try to further strengthen Pakistan-US ties.
Until his appointment as Islamabad's envoy in Washington this month, Jilani was serving as Pakistan's foreign secretary.
Earlier, Jilani served in the Pakistan embassy in Washington from 1995 to 1999. His previous assignments include deputy high commissioner in New Delhi from 1999 to 2003 and envoy to European Union and Australia.
US-Pakistan ties have been under strain since May 2011, when US Navy SEALs swooped down on a compound in Abbottabad and killed Al-Qaeda chief Osama bin Laden who was in hiding there.
The killing of two Pakistani men by Central Intelligence Agency contractor Raymond Davis and a US air raid that killed 28 Pakistani soldiers further strained relations.