Prime Minister Lee Hsien Loong sent a condolence letter to the widow of Mr Lee Khoon Choy yesterday, saying the former People's Action Party (PAP) old guard MP was steadfast and loyal till the end.
He said founding Prime Minister Lee Kuan Yew once described Mr Lee as "a man who 'had it', a man of courage who did not melt under pressure".
Mr Lee, aged 92, died last Saturday of pneumonia at the National University Hospital.
In the three-page letter to Madam Eng Ah Siam, PM Lee noted that the late Mr S. Rajaratnam, Singapore's first foreign minister, recruited Mr Lee to join the PAP.
Mr Lee, known as KC to friends, was then a journalist, and had covered major events that affected Singapore and the region, such as the 1956 Merdeka talks in London to discuss self-government in Singapore.
After he joined the PAP in 1959, he was elected the Legislative Assemblyman for Bukit Panjang.
In the 1960s, pro-communist factions in the PAP that later broke away to form Barisan Sosialis tried to woo Mr Lee over, said PM Lee.
When they failed, they sent him a death threat in a letter with a bullet enclosed. In 1961, thousands of Chinese secondary school students also staged sit-ins where they threw rotten apples at him and shouted "Lee Khoon Choy, enter the coffin" in Mandarin.
"But KC did not buckle. He was steadfast and loyal till the end," said PM Lee.
In the 1963 elections, Mr Lee lost his seat. But he won a 1965 by-election in Hong Lim - triggered by the resignation of the PAP's Ong Eng Guan - that decided the future of Singapore, said PM Lee.
Singapore, part of Malaysia then, was campaigning for a Malaysian Malaysia, which was anathema to the federal government in Kuala Lumpur, said PM Lee.
The federal government was testing the PAP's support and the outcome of the "pivotal election" would determine "whether they could take over Singapore or kick us out of Malaysia".
A month after Mr Lee's win, Singapore separated from Malaysia.
As ambassador to Indonesia in the 1970s, Mr Lee also "played a crucial role in thawing relations between Singapore and Indonesia", said PM Lee.
Singapore had hanged two Indonesian commandos who bombed the MacDonald House in Dhoby Ghaut in 1965 and killed three people.
Mr Lee, with his good understanding of the Indonesian psyche, persuaded Mr Lee Kuan Yew to make a conciliatory gesture of scattering flowers at the graves of the two commandos in Jakarta.
"The gesture enabled Mr Lee Kuan Yew to establish a relationship with President Suharto and set Singapore and Indonesia on a path of friendship and cooperation," said PM Lee.
In recognition of his contributions, Mr Lee was awarded the Distinguished Service Order by the Government in 1990.
After retiring from politics, Mr Lee went back to his "first love" of writing and painting, PM Lee said.
One of his books, Golden Dragon And Purple Phoenix, which PM Lee launched in 2013, traced the history and influence of inter-marriages between Chinese immigrants and South-east Asian natives.
PM Lee said Mr Lee had "always been interested in where we come from", and believed that people must know their roots.
"Indeed, as we remember his life, we remember our roots and what our pioneers like him did, to put Singapore on the path it is on today."
Mr Lee is survived by his wife, seven children and 11 grandchildren. His wake will be held at the Mount Vernon Sanctuary today and tomorrow.