With total traffic of 75 million, Tokyo's Haneda Airport rounded out the top five.
Heathrow's 74.9 million passenger tally put it in sixth spot, narrowly beating Los Angeles International, which welcomed 52,000 fewer travellers.
Heathrow had the second-most international passengers (69 million), trailing only Dubai (77 million).
In the overall table, Hong Kong (68 million), Paris' Charles de Gaulle (65 million) and Dallas/Fort Worth (64 million) placed eighth through ninth, respectively, as passenger traffic around the world increased by 6.1 per cent.
Atlanta is constantly on top of the table thanks to its strategic location as a connecting hub for flights to North America and is within a two-hour flight of 80 per cent of the US population.
It serves as the base for Delta Air Lines, one of the world's largest carriers.
Dubai has made huge gains after boosting capacity and becoming an important hub for flights between east and west, while Chicago O'Hare has improved its position also after increasing capacity.
ACI said its final report will be published this summer and will include more than 1,900 airports.
Rank (2014 rank)
1 (1) Atlanta (ATL), 101.4 million passengers
2 (2) Beijing (PEK), 89.9 million passengers
3 (6) Dubai (DXB), 78 million passengers
4 (7) Chicago (ORD), 76.9 million passengers
5 (4) Tokyo (HND), 75.3 million passengers
6 (3) London (LHR), 74.98 million passengers
7 (5) Los Angeles (LAX), 74.93 million passengers
8 (10) Hong Kong (HKG), 68.2 million passengers
9 (8) Paris (CDG), 65.7 million passengers
10 (9) Dallas/Fort Worth (DFW), 64 million passengers
- Daily Mail