"It is difficult to imagine that so much time has passed," Ms Rao told a select audience gathered at the Foggy Bottom headquarters of the US State Department, which hosted a rare farewell reception for the Indian diplomat on the last day of her assignment on Monday night.
Ms Rao, 62, served as India's Foreign Secretary, External Affairs Ministry's spokesperson and envoy to important countries like China and Sri Lanka in four decades as a diplomat.
"I leave with a great sense of optimism and hope and satisfaction that I have been able not only to learn so much but also to help build partnerships and forge relationship between India and a number of countries around the world," said Ms Rao.
US Secretary of State John Kerry sent a special message to Ms Rao which was read on the occasion.
He appreciated Ms Rao's role in strengthening the important partnership between India and the US. "I wish you all the best in the day, months and years to come," Mr Kerry said in the message.
"An honour to be hosted to farewell @StateDept today.
Touched by Secretary Kerry's message and presence of so many friends and colleagues," Ms Rao tweeted after the reception. Ms Rao is one of the few diplomats who has over 190,000 followers on the microblogging site.
At the reception, she was described as "tireless champion" of Indo-US Strategic Partnership and that during her two-year tenure, the bilateral relationship has reached new heights.
Last week, Deputy Secretary of State William Burns hosted a luncheon for her at the State Department.
The Indian-American community and the US India Business Council also hosted receptions for her last week.
Ms Rao also hosted a farewell reception at her residence last week, which was attended by top American diplomats and members of the Indian-American community.
Among those in attendance at the reception were US Assistant Secretary of State for South and Central Asia Nisha Biswal and her predecessor Robert Blake; and US Under Secretary of State for Management Patrick Kennedy.
Ms Rao will be succeeded by S Jaishankar, currently India's Ambassador to China.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.