The AW101 entered into service in 1999 and has since replaced several older helicopter types, such as the Sikorsky SH-3 Sea King, performing roles such as medium-sized transport, anti-submarine warfare, search and rescue, and ship-based utility operations.

Another variant, the Lockheed Martin VH-71 Kestrel, was produced to serve in the United States presidential transport fleet before the program was cancelled and the aircraft sold off to Canada for parts.

In 1977, the UK Ministry of Defence issued a requirement for an anti-submarine warfare (ASW) helicopter to replace the Royal Navy's Westland Sea Kings, which were becoming inadequate in the face of advances in Soviet submarine technology.

[7] Agusta and Westland finalised an agreement to work on the project together, and formed a jointly owned new company, EH Industries Limited (EHI), to pursue the development and marketing of the new helicopter to potential operators.

On 12 June 1981, the UK government confirmed its participation in the project, allocated an initial budget of £20 million to develop nine pre-series examples.

[11] The fledgling EH101, of which up to 50 were on order to replace the Canadian Armed Forces's (CAF) Sea Kings, found itself subject to a wider political battle between the country's Conservative and Liberal parties, the latter viewing the aircraft as too costly.

[24] The EH101 order was not without controversy, the RAF had declared its preference for an all-Chinook fleet; also, Boeing allegedly offered cheaper terms for the Chinook.

[33] By September 2013 AgustaWestland was in the process of acquiring civil certification for the AW101; prospective customers include offshore oil platform operators and VIP clients.

[55] Two hardpoints are present in the underside of the airframe on which the HM1 model can carry four Sting Ray torpedoes or Mk 11 Mod 3 depth charges.

[57] The Mk1, Mk3 and Mk3A variants can mount general purpose machine guns in up to five locations in the main cabin, aimed out of both door and window apertures.

[40][63] As part of the Royal Navy's Merlin HM2 upgrade program, Lockheed Martin implemented a series of improvements to the radar, notably allowing it to track 40 times the number of targets previously capable.

[60] Mk 2 Royal Navy Merlins are equipped with the AQS903 anti-submarine system for processing sonographic data from sonobuoys to detect and target submerged submarines.

[72] In March 2004, Navy Merlins were temporarily grounded following an incident at RNAS Culdrose in which a tail rotor failed due to a manufacturing defect.

[75] However, the subsequent Strategic Defence and Security Review 2010 stated that the future naval helicopters would be the Merlin and the Wildcat, a derivative of the Lynx.

[83][84] In addition to its ASW role, the HM2 will be able to carry an airborne early warning (AEW) pod known as the Crowsnest, replacing the Sea King ASaC7.

[78][87][88] On 22 May 2015, the MOD and prime contractor Lockheed Martin UK selected Thales to provide the radar and mission system for the Crowsnest.

In 2021, the UK's Defence and Security Accelerator (DASA) announced that options would be examined and might draw on the already existing Project Vixen, researching the utility of a naval unmanned system that could encompass strike, AEW, and air-to-air refuelling components.

[105] In 2002, Westland made an unsuccessful unsolicited offer to the British Ministry of Defence, proposing an enhanced Merlin variant intended to satisfy the demand for additional lift capacity.

[108] The first HC4 began flight trials in September 2017, with an IOC in mid-2018;[108] an interim folding main rotor head will be fitted to some HC3 for shipborne operations prior to the HC4 upgrade.

[119] In June 2011 the AW101 was chosen by the Italian Air Force to replace its ageing Sea King HH-3F helicopters in the combat search and rescue role.

[127] In 2001, the Royal Danish Air Force (RDAF) announced the purchase of eight EH101s for SAR duties and six tactical troop transports for 722 Squadron.

[129] Danish AW101s have a higher gross weight capacity of 15,600 kg and were, as delivered, coated in a paint designed to reduce the aircraft's infrared signature.

[132] In the first six months of 2008, the RDAF reported an operational availability of roughly 50%, well below the target of 80%, partly due to an inadequate maintenance organisation and staff shortages.

[134][135] In February 2013, Aviation Week reported that earlier reliability problems had been resolved and that a full upgrade package would be applied to Danish AW101s; these included the addition of electronic warfare pods and a new electro-optical system, in advance of a deployment in Afghanistan in 2014.

[137] On 12 June 2017, Per-Willy Amundsen, Minister of Justice and Public Security, announced the opening of Leonardo's AW101 Norway Training Centre at Stavanger Sola Air Station.

[141] The Sea King was phased out in December 2023, after 47 years of service as rescue helicopter in the Royal Norwegian Air force, and replaced with the AW101.

They are operated by 751 Squadron and are kept on constant alert at three bases: Montijo near Lisbon, Lajes Field on the Azores, and Porto Santo Island.

[146] The Japan Maritime Self-Defense Force (JMSDF) ordered 14 aircraft in 2003 to use in both the airborne mine countermeasures (AMCM) and transport roles.

The CH-101 will operate in the transport/support role for Antarctic expeditions, replacing the Sikorsky S-61A, and will be working in coordination with the Ministry of Education, Culture, Sports, Science and Technology.

[153][154] AgustaWestland developed a specialised luxury variant, the AW101 VVIP (Very Very Important Person, i.e. a head of state), aimed at business and VIP customers.