Annie Haslam and Phil Everly contributed guest vocals to the album; Wood's influences on the record included the Andrews Sisters, the Beach Boys and Led Zeppelin.

In 1973, Roy Wood released his debut solo album Boulders, on which he performed virtually every instrument,[2] and achieved hits with his band Wizzard, including the UK number one singles "See My Baby Jive" and "Angel Fingers".

"[6] In the run-up to the release of the album, Wood fulfilled a long-time desire when he began using the bagpipes in his work, including onstage where he tuned the drones in order to play Scottish marches, although he found the instruments difficult to maintain due to different temperatures affecting their pitch, and also punctured a set.

[8] Wood explained that he believed solo albums should feature a sole performer, as opposed to artists who hire session musicians like "flippin' Leon Russell and so on" to contribute ideas.

[11] Wood achieved the recording of what resembles a choir by playing "a sort of musical chairs", according to Jack Lewis of the Daily Mirror.

[6][13][14] "Any Old Time Will Do" is a straightforward, melodic song,[5] while "The Rain Came Down on Everything" is a ballad with a medieval, quasi-classical style, featuring an atmospheric intro of choral singing and eerie noises and a crescendo with Wagnerian-style operatic vocals, harmonies, harps and heavy drums, eventually culminating in the sound of a thunderstorm.

[13] Writer Philip Auslander felt the song's opening section, with its classical-styled passage and electronic sounds reminiscent of busy telephone signals, proved Wood was "welcoming us into a chaotic realm where anything is possible", noting the diverse usage of both musical and noise elements.

[14] Although the studio dispute delayed release of Mustard,[6] Wood had been discussing the album in interviews for almost two years before its eventual appearance.

[15] Ray Fox-Cumming of Record Mirror & Disc said the single was "lost in the Christmas rush" and could have been more successful if Wood performed it on Top of the Pops.

[15] Despite the album's under-performance, Wood told Fox-Cumming in a March 1976 interview that he considered Mustard to be better than Boulders, as well as containing stronger production.

[22] Alan Betrock of Phonograph Record felt the album was "a highly respectable effort" that was perhaps indicative that Wood "may be leaving his roots behind and once again planning to create adventurous and original modern pop".

[23] Peter Harvey of Record Mirror & Disc hailed Wood's multiple talents but felt he worked too hard on Mustard, calling it "a complex mish-mash of styles and ideas which only add up to the eccentric experimenting of a studio/musical dilettante".

[13] In a review of the Edsel reissue, Stephen Thomas Erlewine of AllMusic hailed Mustard as "a shining, glittering pop record" with an attractively smooth flow, and concluded that "Mustard might not equal the brilliantly maverick Boulders, yet it's easily one of the best, most cohesive records Wood ever made and one of the few to capture him as a (relatively) focused pop craftsman.

[26] Colin Larkin, writing in The Virgin Encyclopedia of Popular Music, considers both albums to be "uneven", but notes how they revealed Wood's "surplus creative energies as a multi-instrumentalist, engineer, producer and even sleeve designer".