With the onset of the First World War supplies of the Marklin clockwork motors from Germany naturally ceased - an alternative US-sourced 4 volt electric motor was soon provided, and the first electric Meccano motor operating directly from the mains also appeared at this time. Apart from cosmetic changes in the finish of some parts, and alternatives for brass bosses in smaller pulleys, wartime production appears to have continued without undue hindrance. The first Book of Prize Models appeared in 1915, along with the Inventor's Accessory Outfit. In 1916 the first "Meccanograph" appeared as a competition entry - it became one of Frank Hornby's favourite models, and Meccanographs remain popular with modelbuilders today. A bi-monthly Meccano Magazine began with the free September/October issue of 1916. The first Curved Strips and the special loom parts appeared during this time, and Braced Girders finally eliminated the need for prodigious amounts of Cord for bracing purposes. Hornby formed the Meccano Guild in 1919: its first affiliate was the Holy Trinity Club in Kent, which prospers still. UK production of the No. 1 clockwork motor began in 1919, along with the first (constructional) Hornby trains. The Meccano Magazine doubled its size to eight pages, and now cost one penny: it was becoming an effective means of communication between Meccano Ltd. and Meccano boys at large. New parts were advertised for the first time in the Meccano Magazine in 1920, and a column for suggested new parts began in 1921.

Electrical parts first appeared in 1920, the Electrical Accessory Outfit containing 15 new parts, as well as an electric motor and accumulator. The first vertical steam engine appeared - its very high price helped to ensure that it soon disappeared again! By 1921 most Angle Girders were available - in view of the large skeletal models up to this time, the extra Girders were a long time in coming, and accounted for the relatively large numbers of strips and angle brackets in earlier sets. Many of the more specialised parts such as Flywheels, Rack Segments and Dredger Buckets appeared during 1921, and in 1922 a Meccano constructional Crystal Set went on sale. "Fabrique en Angleterre" markings on imported Meccano parts were required by French law and date from late 1921 until 1926.

1922 saw the introduction of the famous No. 7 Meccano Outfit. (This outfit formed the basis of what became the L Set of the 1930s and subsequently the No. 10 Set.) With it, many of the major prize-winning models could be built, and it included special loom parts, the complete electrical outfit, a 4-volt electric motor and accumulator and a clockwork motor. Housed in a stained oak cabinet, the No. 7 was the pinnacle of Meccano ownership, and quite unaffordable to most. The Meccano Magazine appeared monthly from September 1922. The 7/32" long bolt was first introduced in 1922 in place of the earlier 1/4" - and longer - bolts, but it was not until 1925 that this became standard, thus solving problems of interfering bolts in tight places. The Hub Disc, Circular Girder and Channel Segment appeared in late 1923. Full-colour covers graced the Meccano Magazine from the May 1924 issue. 1925 saw an increase in the size of the No. 7 Outfit and a 96 page December issue of the sixpenny Meccano Magazine. The first painted "New" Meccano parts appeared in 1926 in bright red and pea-pod green, a colour scheme which was mercifully very short-lived. Several new parts during these years brought the total number of parts in the Meccano system, including electrical parts and motors, to nearly 240 by 1926.