After losing 15-11 to a Queensland side in Brisbane that had 11 Wallabies in its starting XV he went 23 straight games without defeat before drawing his final game for the Lions in South Africa three years later.
"Before we got to New Zealand on the 1971 tour we had two games in Australia. We opened the tour against Queensland and then faced New South Wales," recalled Slattery at an HSBC Lions Legends dinner in Dublin.
"I don't think that if we had played as well as any Lions team could have done we would have won that game in Brisbane. The penalty count against us was 15-3 in Queensland and then 21-3 in New South Wales.
"It was incredible. We managed to beat New South Wales 14-12, but if we could make that many mistakes in those two games and then go to New Zealand and go unbeaten in all our provincial matches it either says something about our team, or the referees."
Fergus Slattery first became a Lion on the 1971 tour of Australasia
After that losing start, Slattery became a core member of the Lions squad that became the only British & Irish team to win a Test series in New Zealand. Although he didn't get a run in the four Tests, he played on 12 winning teams in provincial fixtures.
From there he moved on to South Africa with the all-conquering 1974 Lions, this time becoming a key member of the most feared pack the Lions have ever fielded.
The Springboks were swamped 3-0 in the four-game series, with the last match being drawn.
That was the only game the Lions didn't win and Slattery scored six tries in 12 outings. And there could so easily have been a seventh, crucial try in the fourth and final Test at Ellis Park as the Lions drove over the line. Slattery appeared to get the ball down for a try that would have broken the deadlock at 13-13, but South African referee Max Baise didn't award it.
"The referee is always right even if he's wrong," said Slattery. "But I don't think that was the reason why we didn't win that final Test.
"As a side, we played as though we had packed our bags to go home on the Thursday. I certainly felt that and I think we were probably a bit guilty of pushing that fourth Test down the agenda a little bit.
"We had a couple of injury problems and perhaps we didn't give the game the full attention it deserved at the end of a long, but otherwise successful tour."
The 1971 and 1974 tours produced the only back-to-back series victories the Lions have enjoyed since their early touring days in the 19th century with five wins and two draws in eight tests against the full might of the All Blacks and Springboks.
So what made those Lions so special?
"We were all so fortunate to have been around together at the same time. When you look at the quality of the players we had to choose from then it was superb," added Slattery.
"If you picked an all-time Lions XV you would probably find it dominated by the players of that era."
Twice a Lions tourist, Slattery wore the green of Ireland on 61 occasions
Slattery's Lions lowdown
John Fergus Slattery
Born: Dún Laoghaire, 12 February, 1949
Irish Caps: 61 (18 as captain)
Honours: 1982 Triple Crown; International Rugby hall of Fame Inductee 2007
British & Irish Lions: 25 games (W23 D1 L1); Tests: P4 W3 D1
1971 IN NEW ZEALAND
12/05/1971 Queensland 15 - 11 Lions
26/05/1971 Wanganui/King Country 9 - 22 Lions
29/05/1971 Waikato 14 - 35 Lions
05/06/1971 Wellington 9 - 47 Lions
16/06/1971 West Coast/Buller 6 - 39 Lions
19/06/1971 Canterbury 3 - 14 Lions
30/06/1971 Southland 3 - 25 Lions
03/07/1971 Taranaki 9 - 14 Lions
06/07/1971 NZ Universities 6 - 27 Lions
14/07/1971 Wairarapa/Bush 6 - 27 Lions
17/07/1971 Hawkes Bay 6 - 25 Lions
24/07/1971 Auckland 12 - 19 Lions
10/08/1971 Bay of Plenty 14 - 20 Lions
1974 IN SOUTH AFRICA
18/05/1974 South West Africa 16 - 23 Lions
26/05/1974 Eastern Province 14 - 28 Lions - Try
29/05/1974 South Western Districts 0 - 97 Lions - Try
04/06/1974 SAR Federation XV 6 - 37 Lions - Try
08/06/1974 South Africa 3 - 12 Lions
18/06/1974 Rhodesia 6 - 42 Lions - Try
22/06/1974 South Africa 9 - 28 Lions
29/06/1974 Orange Free State 9 - 11 Lions
03/07/1974 Griqualand West 16 - 69 Lions - Try
13/07/1974 South Africa 9 - 26 Lions
20/07/1974 Natal 6 - 34 Lions - Try
27/07/1974 South Africa 13 - 13 Lions