Lochner worked in the late International Gothic (schöne stil) style, already considered dated and old-fashioned by the 1440s, yet is widely regarded as innovative. He introduced a number of progressions to painting in Cologne, especially by filling his backgrounds and landscapes with specific and elaborate details, and by rendering his figures with more bulk and volume. Wellesz described his paintings as evidencing an "intensity of feeling which gives a very special and very moving quality to his work. His devotion is reflected in his figures: it charges with symbolic meaning the smallest details of his paintings; and, in a hidden, almost magical way, it speaks from the concord of his pure and glowing colours."
Lochner painted with oil, preparing the surface in a way typical of other North German artists; in some works, he attached canvas to the panel support underneath the usual chalk ground. This was probably done where there were to be large areas of plain gilding. Where the gold ground was to have a pattern such as a brocade, this was carved into the chalk ground before gilding, and, in some paintings, elements had moulded additions applied to raise the surface to be gilded. He employed a number of techniques when gilding, to give different effects. These included laying the leaf with water for burnished passages, and with oil or varnish sizing (mordant gilding) for the more decorative areas. His colour schemes tend to be bright and luminous, filled with varieties of red, blue and green pigments. He often employed ultramarine, then expensive and difficult to source. His figures are regularly outlined with red paint. He was innovative in his rendering of flesh tones, which he built up using lead whites to give pale complexions with almost porcelain qualities. In this, he refers to an older tradition of indicating women of high nobility whose paleness was associated with a life spent indoors, "shielded from toiling in the fields, which was the lot of most". In particular, this technique follows the Master of Veronica, although the earlier painter's figures had an almost yellowish, ivory hue. Lochner's Madonnas tend to be clothed in saturated blues which resonate with surrounding yellow, red and green paint. According to James Snyder, the artist "employed these four basic colors for his harmonies", but went beyond by using more subdued and deep hues in a technique referred to as "pure color".
Like Conrad von Soest, Lochner often applied black cross-hatching on gold, usually to render metallic objects such as brooches, crowns or buckles, in imitation of goldsmiths work on precious objects such as reliquaries and chalices. He was heavily influenced by the art and process of metalwork and goldsmithing, especially in his painting of gold grounds, and it has been suggested that he may have once trained as a goldsmith. Evidence of his imitation of elements of their craft is apparent even in his underdrawings. Notable and elaborate painted examples include the tooled gold border of the angelic concert in his Last Judgement, and Gabriel's clasp on the outer wing of the Dombild altarpiece.
Lochner seems to have prepared on paper before approaching his underdrawings; there is little evidence of reworking, even when positioning large groups of figures. Infrared reflectography of the underdrawings for the Last Judgment panels show letters used to denote the final colour to be applied, for example g for gelb (yellow) or w for weiss (white), and there are few deviations in the finished work. He often rearranged drapery fold lines or to denote perspective, enlarged or diminished the size of figures. The underdrawings reveal a draughtsman of skill, dynamism, and confidence; the figures appear fully formed with little evidence of reworking. Many are extremely detailed and precisely modelled, for example, St Ursula's brooch in the Altarpiece of the City Patron Saints, which contains closely detailed garlands and diadems.
Saints Mark, Barbara and Luke
, c. 1445–50. 100.5 x 58cm. Wallraf-Richartz Museum, Cologne
Perhaps influenced by van Eyck's Madonna in the Church, Lochner closely detailed the fall and gradient of light. According to the art historian Brigitte Corley, the clothes of "protagonists change their hues in delicate reaction to the influx of light, reds being transformed through a symphony of pink tonalities to a dusty greyish white, greens to a warm pale yellow, and lemon shading through oranges to a saturated red". Lochner employed the notion of supernatural illumination not just from van Eyck, but also from von Soest's Crucifixion, where light emanating from Christ dissolves around John's red robe, as yellows rays eventually become white. There is a real possibility that a number of the faces of saints are modelled on historical persons, i.e. as donor portraits of the commissioners and their wives. Figures fitting this theory include St Ursula and St Gereon panels from the City Saints altarpiece.
Unlike the painters in the Low Countries, Lochner was not so concerned with delineating perspective; his pictures are often set in shallow space, while his backgrounds give little indication of distance and often dissolve into solid gold. Thus, and given his harmonious colour schemes, Lochner is usually described as one of the last exponents of the International Gothic. This is not to say his paintings lack contemporary northern sophistication; his arrangements are often innovative. The worlds he paints are hushed, according to Snyder, achieved with the symmetry of subdued use of colour and the often repeated stylistic element of circles. Angels form circles around the heavenly figures; the heavenly figures' heads are highly circular and they wear round haloes. According to Snyder, the viewer is slowly "drawn into empathy with the revolving forms".
Because of the paucity of surviving attributed works, it is difficult to detect any evolution in Lochner's style. Art historians are unsure if his style became progressively more or less influenced by Netherlandish art. Recent dendrochronological examination of attributed works indicate that his development was not linear, suggesting that the more advanced Presentation in the Temple is of 1445, predating the more Gothic Saints panels now divided between London and Cologne.