1. LAX-SIN (37/38) - 8770m (cancelled 6/2018)
2. IAH-SYD (100/101) - 8596m
3. SFO-SIN (1/2 and 28/29) - 8446m
4. EWR-HKG (117/116 179/180) - 8060m
5. LAX-MEL (98/99) - 7921m
6. EWR-BOM (48/49) - 7806m
7. ORD-HKG (895/896) - 7793m
8. LAX-SYD (839/840) - 7487m
9. SFO-TLV (954/955) - 7,422m
10. SFO-SYD (863/864) - 7419m
11. EWR-PVG (86/87) - 7386m
12. EWR-DEL (82/83) - 7323m
13. IAD-DXB (976/977) - 7069m (cancelled 12/2015)
14. ORD-PVG (835/836) - 7056m

Note: Graphics are in some cases outdated as of Dec. 2018 (routes/aircraft) Top Longest Non-Stop United Flights

1. San Francisco, CA-SFO --> Singapore-SIN takes place as the new #1 longest United route with 8,446 miles (13,593 km) aboard a new B787-9. The route, UA flight # 1 or UAL1, is scheduled at 16 hr 35 min. Service began on June 1, 2016, and United is tied with Singapore Airlines for 3rd longest flight in the world on this route.
2. Newark, NJ-EWR –> Hong Kong-HKG is currently 2nd longest United flight in the world at 8,065 miles (12,979 km). The 15 hr 50 min flight is operated by United’s B777-200. UAL179 (UA flight # 179) is also tied with Cathay Pacific for the 14th longest flight in the world.
3. Los Angeles-LAX –> Melbourne, Australia-MEL clocks in as United 3rd longest flight. Scheduled service began on October 26, 2014 and was made possible by the airline’s shiny new Boeing 787-9. UA flight #98 (UAL98) will take 15 hr 45 min to cover 7,921 mi (12,748 km). Shared with Qantas, this is currently the 23rd longest flight in the world.
4. Newark, NJ-EWR –> Bombay, India-BOM has a scheduled duration of 15 hr 50 min for the 7,807 mile (12,564 km) journey. Coded as UAL49 (UA Flight # 49), the flight is operated by a Boeing 777-200. This is also the 26th longest flight in the world, shared with Air India.
5. Chicago, IL – O’Hare-ORD –> Hong Kong-HKG is also operated by a B777-200. UA flight # 895 covers 7,794 miles (12,543 km) during the 15 hr 55 min flight. Cathay Pacific is tied with United for this route in sharing the 27th longest flight in the world.
6. Los Angeles-LAX –> Sydney, Australia-SYD, UA flight # 839 (UAL839) scheduled flight time is 15 hr 05 min. A Boeing 787-9 covers the 7,488 mile (12,051 km) route.
7. Tel Aviv-TLV –>San Francisco, CA-SFO became the 7th longest United flight when it began service on March 30, 2016. The 7,422 mile (11,936 km) flight is operated by a B787-9. The route, UA flight # 955 or UAL955, is scheduled at 15 hr 05 min.
8. San Francisco, CA-SFO –> Sydney, Australia-SYD comes in at number 6 with 7,417 miles (11,936 km) aboard a B787-9. The route, UA flight # 863 or UAL863, is scheduled at 14 hr 55 min.
9. Newark, NJ-EWR –> Shanghai, China-PVG covers a distance of 7,384 miles (11,883 km) during the 14 hr 55 min flight. UA flight # 86 (UAL86) is flown on a, you guessed it, B777-200.
10. Newark, NJ-EWR –> Delhi, India-DEL, aka UA flight # 83 (UAL83) makes for the 2nd non-stop US to India destination coming in at 7,324 miles (11,787 km). A B777-200 operates the 15 hr 15 min flight.
11. Chicago, IL – O’Hare-ORD –> Shanghai, China-PVG breaks up the B777-200 trend and finally gives us a route operated by a Boeing 747-400. UA flight # 835 (UAL835) lasts appx 14 hr 10 min covering 7,057 miles (11,357 km).
12. San Francisco, CA-SFO –> Hong Kong-HKG aka UAL869 (UA flight # 869) is the second (and final) B747-400 route on this list at 6,927 miles (11,148 km). The flight is scheduled to take 14 hr 15 min.
13. Washington, D.C. – Dulles-IAD –> Beijing, China-PEK has a scheduled flying time of 14 hr 00 min under UA flight # 807 (UAL807). The 6,921 mile (11,138 km) route is operated by a Boeing 777-200.
14. San Francisco, CA-SFO –> Chengdu, China-CTU rounds out the (self-admittedly arbitrarily) cutoff list at number 14. It is also United’s 6th longest Boeing 787 Dreamliner flight at 14 hr 20 min. UA flight # 9 (UAL) covers 6,870 miles (11,056 km).
Other notable mentions

15. Newark, NJ-EWR –> Beijing, China-PEK. The 6,831 miles (10,993 km) long route is coded as UA flight # 89 (UAL89) with a duration of 13 hr 50 min aboard the B777-200.
The next 3 are grouped together since they’re all United hubs to Tokyo-NRT. I find it pretty interesting that all of the flights are within about 100 miles of each other:

16. Washington, D.C. – Dulles-IAD –> Tokyo, Japan-NRT = UA flight # 803 (UAL803) and covers 6,753 miles (10,868 km) over 14 hr 05 min on a B777-200.
17. Newark, NJ-EWR –> Tokyo, Japan-NRT = UA flight # 79 (UAL79) and covers 6,732 miles (10,834 km) over 14 hr 00 min on a B777-200.
18. Houston, TX-IAH –> Tokyo, Japan-NRT = UA flight # 7 (UAL7) and covers 6,643 miles (10,691 km) over 13 hr 45 min on a B777-200.
19. Chicago, IL – O’Hare-ORD –> Beijing, China-PEK = 6,579 mi (10,588 km)

Discontinued longest non-stop United Flights

The only discontinued ultra-long haul United flight I could find data was JFK-HKG, if you know of any others, please post them in the comment section below!

New York, NY – Kennedy-JFK –> Hong Kong-HKG was one of the shortest lived ultra-long haul flies in history. UA Flight #821 (UAL821) began service on April 1, 2001 and was cancelled within 5 months. The 8,072 mile (12,991 km) flight held the record as the longest flight in the world when it first began.Those that flew this rare route spent appx 15 hr 40 min aboard a Boeing 747-400.
Washington, D.C. – Dulles-IAD –> Dubai, United Arab Emirates-DXB used to be United’s 9th longest flight before it was cancelled on Jan 23, 2016. When operating, it was scheduled to take 14 hr 25 min aboard a Boeing 777-200. UA flight # 977 (UAL977) covered a distance of 7,069 miles (11,376 km). Interestingly, this was the only route that United fly to Dubai-DXB and now there are none.related: https://www.flyertalk.com/forum/united-airlines-mileageplus/1543130-shortest-united-flight-you-can-purchase.html