The line has a total length of 48 km (29.83 mi), operating as a semi-circle or spiral route from Lak Song to Tha Phra which also serves as a self-interchange station.

The line was extended to the west from Hua Lamphong through new stations in Bangkok Chinatown and Ko Rattanakosin on 29 September 2019.

Another 7.8 km (4.8 mi), 4 station extension from Lak Song to Phutthamonthon Sai 4 is planned but remains unbuilt.

Construction of the line began on 19 November of that year, when Crown Prince His Royal Highness Crown Prince Maha Vajiralongkorn (later King Vajiralongkorn) came to lay the foundation stone for the construction of the MRT project, which is Thailand's first subway project, with the aim of relieving Bangkok's traffic problems.

[10][11] The project suffered multiple delays not only because of the 1997 economic crisis, but also due to challenging civil engineering works of constructing massive underground structures deep in the water-logged soil upon which the city is built.

The MRT Blue Line opened for a limited public trial period of several weeks starting on 13 April 2004.

On 3 July 2004, the line was officially opened at 19:19 local time by HM King Bhumibol and Queen Sirikit who were accompanied by other members of the royal family.

In August, 2017, BEM awarded the automatic fare collection ticket for the extension to Thales which to install its TransCity system.

[27] Under the new agreement, BEM will operate the new extensions from Hua Lamphong to Lak Song and Bang Sue to Tha Phra.

In July 2014, the MRTA submitted a request for 84 million baht to complete a route survey for a 7.8 km (4.8 mi), 4 station extension from Lak Song to Phutthamonthon Sai 4.

[28] In late March 2017, the Transport Ministry stated that this 7.8 km (4.8 mi) extension would be submitted to Cabinet for approval in April 2017 with construction expected to commence in the 4th quarter of 2017.

[46] In 2017, BEM starts removing some seats from the train to increase interior space to cope with rising number of passengers.

[47] In 2000, Alstom won a contract to supply 21x3 car Metropolis train sets, and this design would have been similar to C751A used for Singapore's North East MRT line if it had been built.

The bidding for rolling stock was conducted again, and bidders include Bombardier Transportation (later acquired by Alstom in 2021) and Siemens, both from Canada and Germany.

* Car 1016 and 1028 were swapped after the train collision in 2005.In 2017, BEM ordered 35 three-car as part of the line extension program.

[52][53] Under the contract, Siemens were to supply 35 trains and provide maintenance for ten years with a price tag of 20 billion baht.

[54] On 18 July 2024, Mr. Phongsarit Tantisuvanitchkul, executive chairman of BEM revealed that the company will order an additional 21 train sets, each comprising three cars, for the MRT Blue line to meliorate congestion during peak hour.

Siemens and ST Electronics will supply the bogies, traction, braking, and auxiliary systems, and will be responsible for project management, development, construction, and commissioning.

During the weekday morning rush hour, there are two service patterns: At opening in 2004, initial ridership was 180,000 per day — considerably lower than the projections of over 400,000.

[2] The MRT Blue line was expected to grow to a daily ridership of 800,000 once the Lak Song and Tha Phra extensions opened.

However, the impact of the COVID-19 pandemic by late 2020 had resulted in a dramatic reduction of ridership down to an average of 360,000 per weekday reverting to September 2017 levels.

This measure aims to address the worsening air pollution caused by surging dust levels across all districts of the capital.

After initial investigations, it was found that the empty train had run into problems shortly before the accident, grinding to a halt on a curve leading to the depot.